ED U CAT2 QNf 


RUDIMENTS 

or 

NATURAL  PHILOSOPHY  AND  ASTRONOMY 

DESIGNED   FOR  THE 

YOUNGER  CLASSES  IN  ACADEMIES, 

AND   FOR 

COMMON  SCHOOLS. 


BY   DENISON   OLMSTED, 

PROFESSOR  OF  NATURAL  PHILOSOPHY  AND   ASTRONOMY 
IN  YALE   COLLEGE. 


STEREOTYPE    EDITION. 


NEW-HAVEX-   S.J3 
NEW  YORK :   COLLINS,  BROTHER,  &  CO., 

254   PEARL-STREET. 

1844. 


£DUC. 


Entered,  according  to  Act  of  Congress,  in  the  year  1844, 

BY  DENISON  OLMSTED, 
In  the  Clerk's  Office  of  the  District  Court  of  Connecticut. 


EDUCATION  DEFT. 


Stereotyped  by 

RICHARD. C.  VALENTINE, 
45  Gold  ffeMti  re  w  Yori. 


PREFACE. 


SOME  years  since,  I  announced  to  the  pnblic  an  intention  of 
preparing  a  series  of  text-books,  in  Natural  Philosophy  and  As- 
tronomy, adapted,  respectively,  to  Colleges,  Academies,  and 
Common  Schools.  A.  Treatise  on  Natural  Philosophy  in  two 
volumes,  8vo,  and  a  Treatise  on  Astronomy  in  one  volume,  8vo, 
a  School  Philosophy,  and  a  School  Astronomy,  each  in  a  duode- 
cimo volume,  have  long  been  before  the  public,  and  have  pass- 
ed  through  numerous  editions.  Various  engagements  have  pre- 
vented my  completing,  until  now,  the  original  plan,  by  adding 
a  work  of  a  form  and  price  adapted  to  the  primary  schools,  and 
in  a  style  so  easy  and  familiar,  as  to  be  suited  to  pupils  of  an 
earlier  age  than  my  previous  works. 

In  writing  a  book  for  the  pupils  of  our  Common  Schools,  or 
for  the  younger  classes  in  Academies,  I  do  not,  however,  con- 
sider myself  as  writing  for  the  ignorant  and  uncultivated,  but 
rather  for  those  who  have  but  little  time  for  these  studies,  and 
who,  therefore,  require  a  choice  selection  of  principles,  of  the 
highest  practical  utility,  and  desire  the  greatest  possible  amount 
of  valuable  information  on  the  subjects  of  Natural  Philosophy 
and  Astronomy,  in  the  smallest  compass.  The  image  which  I 
have  had  constantly  before  me,  is  that  of  an  intelligent  scholar, 
of  either  sex,  from  twelve  to  sixteen  years  of  age,  bringing  to  the 
subject  a  mind  improved  by  a  previous  course  of  studies,  and  a 
capacity  of  being  interested  in  this  new  and  pleasing  depart- 
ment of  knowledge.  I  have  imagined  the  learner,  after  having 
fully  mastered  the  principles  explained  in  the  first  part,  which 
treats  of  Natural  Philosophy,  entering  upon  Astronomy,  in  the 
second  part,  with  a  capacity  much  enlarged  by  what  he  has  al- 
ready acquired,  and  with  a  laudable  curiosity  to  learn  the  se- 
crets of  the  skies.  I  have  imagined  his  teacher  lending  him  oc- 
casional aid  from  a  map  of  the  stars,  or  a  celestial  globe,  and 


stimulating  as  well  as  rewarding  his  curiosity,  by  pointing  out  to 
him  the  constellations.  It  is  hoped,  also,  that  most  of  the  teach- 
ers who  use  this  work,  will  have  the  still  higher  advantage  of 
affording  to  youthful  curiosity  a  view  with  which  it  is  always 
delighted, — that  of  the  moon,  planets,  and  stars,  through  a  tel- 
escope. 

I  should  deem  myself  incompetent  to  write  a  book  like  the 
present,  if  I  had  not  been,  myself,  a  teacher,  first  in  a  common 
school,  and  afterward  in  an  academy  or  grammar  school  of  the 
higher  order.  No  one,  in  my  judgment,  is  qualified  to  write  text 
books  in  any  department  of  instruction,  who  does  not  know,  by 
actual  experience,  the  precise  state  of  mind  of  the  pupils  for  whom 
he  writes.  Several  years  of  experience  in  teaching  the  rudi- 
ments of  knowledge,  in  early  life,  and  the  education  of  a  large 
family  at  a  later  period,  have  taught  me  the  devices  by  which 
the  minds  of  young  learners  are  to  be  addressed,  in  order  that 
subjects  at  once  new,  and  requiring  some  powers  of  reflection  to 
understand  them,  may  be  comprehended  with  perfect  clearness, 
and  of  course  with  lively  pleasure.  Children  are  naturally  fond 
of  inquiring  into  the  causes  of  things.  We  may  even  go  far- 
ther, and  say,  that  they  begin  from  infancy  to  interrogate  nature 
in  the  only  true  and  successful  mode, — that  of  experiment  and 
observation.  With  the  taper,  which  first  fixes  the  gaze  of  the 
infant  eye,  the  child  commences  his  observations  on  heat  and 
light.  With  throwing  from  him  his  playthings,  to  the  great  per- 
plexity  of  his  nurse,  he  begins  his  experiments  in  Mechanics, 
and  pursues  them  successively,  as  he  advances  in  age,  studying 
the  laws  of  projectiles  and  of  rotary  motion  in  the  arrow  and  the 
hoop,  of  hydrostatics  in  the  dam  and  the  water  wheel,  and  pneu- 
matics in  the  wind-mill  and  the  kite.  I  have  in  my  possession 
an  amusing  and  well-executed  engraving,  representing  a  family 
scene,  where  a  young  urchin  had  cut  open  the  bellows  to  find  the 
wind.  His  little  brother  is  looking  over  his  shoulder  with  inno- 
cent and  intense  curiosity,  while  the  angry  mother  stands  be- 
hind with  the  uplifted  rod,  and  a  countenance  which  bespeaks 
the  wo  that  impends  over  the  young  philosopher.  A  more  ju- 
dicious parent  would  have  gently  reproved  the  error  ;  a  more  en- 


lightened  parent  might  have  hailed  the  omen  as  indicating  a 
Newton  in  disguise. 

It  is  earnestly  hoped,  that  the  Rudiments  of  Natural  Philoso- 
phy and  Astronomy, — as  much,  at  least,  as  is  contained  in  this 
small  volume, — will  be  studied  in  every  primary  school  in  onr 
land.  In  addition  to  the  intellectual  and  moral  advantages, 
which  might  reasonably  be  expected  from  such  a  general  diffu- 
sion of  a  knowledge  of  the  laws  of  nature,  and  the  structure  of 
the  universe,  incalculable  advantages  would  result  to  society 
from  the  acquaintance,  which  the  laboring  classes  would  thus 
gain,  with  the  principles  of  the  arts ;  principles  which  lie  at 
the  foundation  of  their  daily  operations, — fora  "principle  in  sci- 
ence is  a  rule  in  art."  Such  a  knowledge  of  philosophical  prin- 
ciples, would  suggest  easier  and  more  economical  modes  of  per. 
forming  the  same  labor  ;  it  would  multiply  inventions  and  dis- 
coveries ;  and  it  would  alleviate  toil  by  mingling  with  it  a  con- 
stant flow  of  the  satisfaction  which  always  attends  a  clear  un- 
derstanding of  the  principles  of  the  arts. 

Although  this  treatise  is  especially  designed  for  schools,  yet  I 
would  venture  to  recommend  it  to  readers  of  a  more  advanced 
age,  who  may  desire  a  concise  and  comprehensive  view  of  the 
most  important  and  practical  principles  of  Natural  Philosophy 
and  Astronomy,  comprising  the  latest  discoveries  in  both  these 
sciences.  The  part  on  Astronomy,  especially,  when  compared 
with  the  sketches  contained  in  similar  works,  may  be  found, 
perhaps,  to  have  some  advantages  in  the  selection  of  points  most 
important  to  be  generally  known — in  perspicuity  of  style  and 
arrangement — and  in  simplicity  and  fulness  of  illustration.  It 
may,  however,  be  more  becoming  for  the  author  to  submit  this 
comparison  to  the  judgment  of  the  intelligent  reader. 
1* 


CONTENTS. 


PART   I. 

NATURAL  PHILOSOPHY.  *•** 

INTRODUCTION. — Grand  Divisions  of  the  Natural  Sciences,  9 

CHAPTER  I. — GENERAL  PROPERTIES  OF  MATTER. 

Extension  and  Impenetrability — Divisibility — Porosity — Com- 
pressibility— Elasticity — Indestructibility — Attraction,  -  19 

CHAPTER  II. — MECHANICS. 

Motion  in  general — Laws  of  Motion — Center  of  Gravity — Prin- 
ciples of  Machinery, -  -  24 

CHAPTER  III. — HYDROSTATICS. 

Pressure  of  Fluids— Specific  Gravity— Motion  of  Fluids— "Won- 
derful Properties  combined  in  Water, 64 

CHAPTER  IV. — PNEUMATICS. 

Properties  of  Elastic  Fluids— Air  Pump— Common  Pump — Sy- 
phon—Barometer— Condenser — Fire  Engine — Steam  and 
its  Properties — Steam  Engine,  -- 84 

CHAPTER  V. — METEOROLOGY. 

General  Objects  of  the  Science — Extent,  Density,  and  Temper- 
ature of  ihe  Atmosphere — Its  Relations  to  Water— Rela- 
tions to  Heat — Relations  to  Fiery  Meteors,  -  -  .  -  -  106 

CHAPTER  VI. — ACOUSTICS. 

Vifaatory  Motion — Velocity  of  Sound — Reflexion  of  Sound — 
Musical  Sounds — Acoustic  Tubes — Stethoscope,  -  119 

CHAPTER  VII. — ELECTRICITY. 

Definitions— Conductors  and  Non-Conductors — Attractions  and 
Repulsions — Electrical  Machines — Leyden  Jar — Electrical 
Light  and  Heat— Thunder  Storms— Lightning  Rods— Ef- 
fects of  Electricity  on  Animals,  .......126 


8  CONTENTS. 

CHAPTER  VIII.— MAGNETISM.  Pa£e- 

Definitions — Attractive  Properties — Directive  Properties — Vari- 
ation of  the  Needle— Dip — Modes  of  making  Magnets,      -  145 

CHAPTER  IX. — OPTICS. 

Definitions— Reflexion  and  Refraction — Colors — Vision— Micro- 
scopes and  Telescopes,         -------  15] 


PART   It. 
ASTRONOMY. 

CHAPTER  I. — DOCTRINE  OF  THE  SPHERE. 
Definitions — Diurnal  Revolutions,       ------  190 

CHAPTER  II. — ASTRONOMICAL  INSTRUMENTS  AND  OBSERVATIONS. 

Telescope — Transit  Instrument — Astronomical  Clock — Sextant,  395 

CHAPTER  III. — TIME.    PARALLAX.     REFRACTION.     TWILIGHT. 

Sidereal  and  Solar  Days— Mean  and  Apparent  Time — Horizon- 
tal Parallax— Length  of  Twilight  in  Different  Countries,  -  202 

CHAPTER  IV.— THE  SUN. 

Distance — Magnitude — Quantity  of  Matter — Spots — Nature  and 
Constitution — Revolutions — Seasons,  -  ...  208 

CHAPTER  V.— THE  MOON. 

Distance  and  Diameter— Appearances  to  the  Telescope — Moun- 
tains and  Valleys— Revolutions— Eclipses— Tides,  -  -  221 

CHAPTER  VI. — THE  PLANETS. 

General  View — Inferior  Planets— Superior  Planets — Planetary 
Motions,  -------._.  235 

CHAPTER  VII. — COMETS. 

Description — Magnitude  and  Brightness — Periods — Quantity  of 
Matter — Motions — Prediction  of  their  Returns — Dangers,  2G2 

CHAPTER  VIII.— FIXED  STARS. 

Number,  Classification,  and  Distance  of  the  Stars— Different 
Groups  and  Varieties — Nature  of  the  Stars,  and  the  System 
of  the  World, -  -  271 


PHILOSOPHY  AND 


PART  I. 

NATURAL  PHILOSOPHY. 
INTRODUCTION.* 

GRAND  DIVISIONS  OF  THE    NATURAL  SCIENCES. 

1.  As  in  Geography  we  have  a  clearer  understanding 
of  particular  countries,  if  we  first  learn  the  great  divi° 
ions  of  the  globe,  so  we  shall  see  more  fully  the  pecu- 
liar nature  of  the  sciences  we  are  now  to  study,  if  we 
first  learn  into  what  distinct  provinces  the  great  empire 
of  science  is  divided. 

To  describe  and  classify  the  external  appearances  of 
things  in  nature,  is  the  province  of  Natural  History  ; 
to  explain  the  causes  of  such  appearances,  and  of  all 
the  changes  that  take  place  in  the  material  world,  is 
the  province  of  Natural  Philosophy.  The  properties 
of  bodies  which  are  presented  to  the  senses,  such  as 
form,  size,  color,  and  the  like,  are  called  external  char- 
acters ;  all  events  or  occurrences  in  the  material  world, 
are  called  phenomena.  Natural  History  is  occupied 


QUESTIONS. 

ARTICLE  1.  What  is  the  province  ol  Natural  History?    Of  Natu- 
ral Philosophy  ?    What  properties  of  bodies  are  called  the  external 
What  are  phenomena  ?    With  what  is  Natural  History 


chwacters  ? 


10  NATURAL  PHILOSOPHY. 

chiofly  .^vjth  ths  external  o-haracters  of  bodies,  which 
it  de'scwjtes  and  cla^>iaes,,  Natural  Philosophy,  with 
phenomena,  which  it  reduces  under  general  laws. 
Ti.ius.  tne  natural , hi stoanu  first  observes  and  describes 
the  external  characters  of  animals,  vegetables,  and  min- 
erals, and  then  classifies  them,  by  arranging  such  as 
resemble  each  other  in  separate  groups.  The  natu- 
ral philosopher,  also,  first  observes  and  describes  the 
phenomena  of  nature  and  art,  and  brings  together  such 
as  are  similar,  under  separate  laws  ;  for  example,  the 
phenomena  and  laws  of  winds,  of  storms,  of  eclipses, 
and  of  earthquakes. 

2.  We  may  form  some  idea  of  the  method  of  classi- 
fication in  Natural  History,  and  of  the  investigation  of 
general  principles  or  laws  in  Natural  Philosophy,  by 
taking  examples  in  each.  The  individual  bodies  that 
compose  the  animal,  the  vegetable,  and  the  mineral 
kingdoms,  are  so  numerous  that,  in  a  single  life,  we 
could  make  but  little  progress  in  acquiring  a  knowl- 
edge of  them,  if  it  were  not  in  our  power  to  collect  into 
large  groups,  such  as  resemble  each  other  in  a  greater 
or  less  number  of  particulars.  When  this  is  done,  our 
progress  becomes  comparatively  rapid  ;  for  what  we 
then  learn  respecting  the  group,  will  apply  equally  to 
all  the  individuals  comprised  in  it.  Hence,  the  various 
bodies  in  the  several  kingdoms  of  nature,  are  distribu- 
ted into  classes,  orders,  genera,  species,  and  varieties. 
Thus,  those  minerals  which  are  like  each  other  in 
having  a  certain  well-known  lustre,  are  collected  to- 
gether into  one  CLASS,  under  the  head  of  Metals,  while 
others  destitute  of  this  peculiar  character,  but  having 
certain  other  characters  in  common,  are  collected  into 


chiefly  occupied  ?  Ditto  Natural  Philosophy  1  Give  an  exam  pie  of  the 
objects  of  the  Natural  Historian.  Also  of  the  Natural  Philosopher. 
2.  "Why  is  it  necessary  to  classify  the  productions  of  nature  ?  How 
does  such  a  classification  make  our  progress  more  rapid  1  Into 
what  are  the  various  bodies  in  nature  distributed  ? 


INTRODUCTION.  11 

another  class,  under  the  head  of  Earths.*     But  some 
metals,  as  lead  and  iron,  easily  rust,  while  others,  as 
gold  and  silver,  do  not  rust  at  all.     Hence,  metals  are 
distributed  into  two  ORDERS  ;  those  which  easily  cor- 
rode being  called  base  metals,  and  those  which  do  not 
corrode,  noble  metals.     But  the  members  of  each  order 
have  severally  distinctive  properties,  which  give  rise 
to  a  further  division  of  an  order  into  GENERA.     Thus, 
iron  constitutes  one  genus  and  lead  another,  of  the  or- 
der of  base  metals.     But  of  each  of  these  genera  there 
are  several  sorts,  as  wrought  iron  and  cast  iron,  white 
lead  and  red  lead.     Each  genus,  therefore,  is  subdivi- 
ded into  SPECIES,  by  grouping  together  such  members 
of  the  same  genus  as  resemble  each  other  in  several 
particulars.     Finally,  the  individuals  of  each  species- 
may  differ  from  each  other,  and  hence  the  'species  is 
still    further  divided    into  VARIETIES.     Thus,   Swedes 
iron  and  Russia  iron,  are  varieties  of  the  same  species 
of  the  genus  wrought  iron,  of  the  order  of  base  metals. 
3.  The  knowledge  we  gain  of  any  individual  i>ody, 
depends  upon  the  extent  to  which  we  carry  the  clas- 
sification of  it.     It  is  something  to  ascertain  the  class 
to  which  ir.  belongs  ;  for  example,  that  the  body  is  a 
metal  and  not  an  earth.     It  is  still  more  to  learn  to- 
what  order  of  metals  it  belongs,  as  that  it  is  one  of  the 
base  and  not  one  of  the  noble  metals.     We  have  ad- 
vanced still  further  when  we  have  ascertained  that  it 
belongs  to  the  genus  iron,  and  not  to  that  of  lead.     If 
we  find  that  it  is  wrought  and  not  cast  iron,  we  ascer- 
tain the  species  ;  and,  finally,  if  we  learn  that  it  is 

*  This  example  is  given  merely  for  the  purpose  of  illustrating  the  method 
of  classification,  and  not  of  showing  the  classification  of  minerals  as  actually 
a'dopted.  This  would  be  too  technical  for  our  present  purpose. 

Give  .an  example  of  classification  in  the  case  of  minerals. 

3.  Upon  what  does  the  knowledge  we  acquire  of  any  individual 
body  depend  1  Show  how  we  proceed  from  the  class  to  the  order, 
from  the  order  to  the  genus,  from  the  genus  to  the  species,  and  from 
the  species  to  the  variety. 


12  NATURAL  PHILOSOPHY. 

Swedes  and  not  Russia  wrought  iron,  we  determine* 
the  variety.  In  regard  to  a  body  newly  discovered, 
whether  an  animal,  plant,  or  mineral,  we  may  gene- 
rally discover  very  readily  to  what  class  and  order  it 
belongs,  but  it  is  usually  more  difficult  to  determine  its 
exact  species  or  variety. 

4.  A  clear  understanding  of  the  method  of  classifi- 
cation employed  in  Natural   History,   will  aid  us  in 
learning  the  method  of  determining  general  principles, 
or  laws,  in  Natural  Philosophy.     A  law  is  the  mode  in 
which  the  powers  of  nature  act ;  and  this  is  determined 
by  the  comparison  of  a  great  number  of  particular 
cases.     Thus,  when  we  have  examined  the  directions 
of  rays  of  light  under  a  great  variety  of  circumstan- 
ces, and  always  found  them  to  be  in  straight  lines,  we 
say  it  is  a  law  of  light  to  move  in  straight  lines.     Laws 
are  more  or  less  extensive,  according  to  the  extent  of 
phenomena  they  embrace.     Thus,  it  is  a  law  of  the 
magnet  that  it  attracts  iron  :  it  is  a  more  extensive  law 
(that  of  gravitation)  that  all  bodies  attract  each  other. 

5.  The  proper  method  of  investigating  any  subject 
in  Natural  Philosophy,  is,  first,  to  examine  with  great 
attention  all  the  facts  of  the  case  ;  secondly,  to  clas- 
sify these,  by  arranging  under  the  same  heads,  such 
as  relate  to  the  same  things  ;  and,  thirdly,  to  state  the 
conclusions  to  which  such   a  comparison  of  the  phe- 
nomena  leads  us.     These  conclusions  constitute  the 
laws  of  that  subject.     Thus,  if  we  apply  heat  to  vari- 
ous bodies,  and  measure  them  before  and  after  heating, 
we  find  in  all  cases  that  their  size  is  enlarged.     Hence 
we  derive  the  law,  that  heat  expands  all  bodies.     If 
we  expose  solid  bodies  to  a  certain  degree  of  heat, 


4.  What  is  a  law  1    How  is  it  determined  1    How  exemplified  in 
the  case  of  light  1    Show  that  laws  may  be  more  or  less  extensive. 

5.  What  is  the  proper  method  of  investigating  any  subject  in  Natural 
Philosophy  1    What  is  the  first  step  ?— the  second  1— -third  1— What 
do  the  conclusions  constitute  ?    Give  an  example  in  the  case  of  heat- 


INTRODUCTION.  13 

they  melt  or  become  liquid,  and  liquids  again  are 
changed  in  the  same  way  to  vapor.  Having  observed 
these  effects  in  a  great  number  of  individual  cases,  we 
lay  it  down  as  a  law,  that  heat  changes  solids  to  fluids 
and  fluids  to  vapors.  By  similar  inquiries  we  ascer- 
tain all  the  laws  of  heat,  which  we  perceive  are,  ac- 
cording to  our  definition,  (Art.  4,)  nothing  more  than 
the  modes  in  which  heat  acts  on  various  bodies.  Laws 
or  general  principles  like  these,  under  one  or  another 
of  which  all  the  phenomena  of  the  material  world  are 
reduced,  constitute  the  elements  of  Natural  Philos- 
ophy. 

6.  The  laws  of  nature,  when  once  learned,  are  ap- 
plied to  the  explanation  of  the  phenomena  of  nature 
or  art,  by  a  process  somewhat  similar  to  that  of  clas- 
sification in  Natural  History.     It  would  afford  a  partial 
explanation  of  the  motion  of  a  steamboat  on  the  water, 
to  refer  it  to  the  general  law  of  elastic  force,  which 
steam  has  in  common  with  air,  and  several  other  nat- 
ural agents  ;  but  it  would  be  a  more  complete  expla- 
nation to  assign  the  particular  mode  in  which  the  force 
acts  upon  the  pistons,  wheels,  and  other  parts  of  the 
machinery.     Science  is  a  collection  of  general  princi- 
ples or  laws  :  Art,  a  system  of  rules  founded  on  them. 
Arithmetic,  so  far  as  it  explains  the  properties  of  num- 
bers, is  a  science  :  so  far  as  it  furnishes  rules  for  the 
solution  of  problems,  or  for  calculation,  it  is  an  art. 
A  principle  in  science,  therefore,  is  a  rule  in  art. 

7.  The  term  "  Natural  Philosophy"  originally  signi- 
fied, the  study  of  nature  in  general.     But  as  the  objects 


What  constitute  the  elements  of  Natural  Philosophy  "\ 

6.  How  are  the  laws  of  nature  applied  to  the  phenomena  of  na- 
ture and  art  ?    What  would  be  a  partial  explanation  of  the  motion 
of  a  steamboat  1     What  would  oe  a  more  complete  explanation  1 
Distinguish  between  science  and  art.     How  far  is  arithmetic  a  sci- 
ence, and  how  far  an  art  1  What  relation  have  the  principles  of  sci- 
ence to  the  rules  of  art  1 

7.  What  did  the  term  Natural  Philosophy  originally  signify  1 

2 


14  NATURAL  PHILOSOPHY"* 

that  fell  under  its  notice  were  multiped,  the  field  be- 
name  too  vast  for  one  mimi.  and  it  was  divided  into  two 
parts — what  related  to  the  earth  h.'longod  to  Natural 
Philosophy,  while  the-  stpn'v  >ily  hodies 

was  erected  into  a  separate  dep  under  the  head 

of  Asti*0tlbig[iy.     By  whole  of 

terrestrial  natu.  were  fur- 

ther inultiplieri,  presented  too  rone  mind 

t<;  explore,   vnd   Natural.  Phi'  ;iras  restricted  to 

the  investigation  of  il";  //-?//•*  re.  while  the  de- 

scription and  classification  of  ,  productions  of  the 
several  kingdoms  of  nature,  w^\  assigned  to  a  distinct 
department  undt-j  u,i  name  of  :  lira)  History.  Still, 
it  was  a  work  tot-  va-s-  to  take  :<i  of  all  the  'phenom- 
ena of  nan  :  ir»i  :u;ate  all  the  laws  that 
govern  them,  ai  'bilosophy  was  again 
divided  int :•  Mtjch&nical  Phil  -phy  and  Chemistry. 
M^chahii -  ;!  l^iilosophy  relates  ,  •  the  phenomena  and 
laws  uf  manses  of  mutter  ;  Chemistry,  to  the  phenom- 
ena and  la.\\vS  of  particles  of  matter.  Mechanical 
Philosophy  considers  those  effects  only  which  are  not 
attended  by  any  change  of  nature,  such  as  change  of 
place,  (or  motion,;  change  of  figure,  and  the  like. 
Chemistry  considers  those  effects  which  result  from, 
the  action  of  the  p&rtidos  of  matter  on  each  other, 
and  which  more  or  less  change  the  nature  of  bodies, 
so  as  to  make  them  something  different  from  what 
they  were  before.  Finally,  it  became  too  much  for 
one  class  of  laborers  to  investigate  the  changes  of  na- 
tuf>e :  or  constitution,  which  are  constantly  going  on  in 
every  body  in  nature,  and  in  every  process,  natural  or 
;ul,  and  Chemistry  was,  therefore,  restricted  to 

Why  was  it  divided  into  two  parts  ?  What  belonged  to  Natural 
Philosophy  1  What,  to  Astronomy  1  How  was  Natural  Philosophy 
ptii.l  lurtner  divided  1  To  whai  was  it  restricted  1  What  was  assigned 
to  Natural  History  ^  Into  what  was  Natural  Philosophy  again  divi- 
ded 1  To  what,  does  Mechanical  Philosophy  relate  1  What  Chemistry  ? 
Wimt  effects  does  Meshaaieal  Philosophy  consider  1  What  Ghent- 


INTKODTTCTTOfl 

inanimate  maj£er,  while/  what.  r^ites  to  i, 
was  erected  into  a  separate  department  undc 
of  Physiology. 

8.  'Natural*  History,  moreover,    found  for  i:-elf  an 
empire  too  vast,  in  attempting  to  ctcscr'be'an''  clas^fy 
the   external    appearances   of   all    thir.g      in   nature. 
Hence  this  study   has  1    en  successiv  )  •    divided  into 
various  departments,  the  study  of  veg"  \bles  being  re- 
ferred to  Botany  •  of  auir-  al>  to"'  Zo    '^y  ;  of  inanimate 
substances  to  Mineralogy.     Still  further  subdivisions 
have  been  introduced   i"to  each  of  these  branches  uf 
Natural  History,  as  :he  o"    ?ct--    embraced  in  it  have 
multiplied.     Thus,  the  stu<      of  that  brarch  of  Zoolo- 
gy   which  relates  to  fi*  ?e  •    i»as    been  erected  into  a 
separate  department  und.     Jhe  head  of  Ichthyology 
of  birds  into  Ornith  •  ogv  ;  and   ct*  insects    Uito    En- 
tomology. 

9.  A   division   of  tb     studies   which  prelate    to   t 
world  we  inhabit,  La:    also  ho.n  made  int<    three  ct 
partments,  Geogrc*.ph~.  ,  Geology,  ami  Met°c  ic-iogy  ;  aT 
objects  on  the  surface  of  the  earth  being  assigned 
Geography  ;  bematii   the    surface,    to    Goology  ;    an 
above  the  surface,  to  Meteorology.      Of  these,  G 
raphy,    in    this    extensive    signification,    present    \\ 
largest  field,  since  it  comprehends,  among  (,,he^  M  ii 
MAN  and  his  works. 

10.  Mechanical    Philosophy    is,    strictl)        -'aki  - 
the  branch  of  human  kijovvledge  which  (\\ 

pose  to  learn  ;  but  it  still  retains  the  or'1 
Natural   Philosophy,   though   in    a    senso 


istry  1    How  was  Chemistry  divided?    T<'  .1  ' 

what  was  assigned  to  Physiology  ? 

8.  Into   what  has  Natural   History  been   snrc^ssively   u 
"What  was  referred  to  Botany?    What  to  Zo<  '    ,     '   A\   lat  i     Min- 
eralogy?   What  further  subdivisions  ijave   b»  -n  intri-du.  •  ••    mto 
each  of  these  branches  1 

9  Into  what  three  departments  has  all  tenv-;i  \\  nature  btf-n  di- 
vided 1  What  isaasianed  to  Geography?  —  what  to  Gevlogyl—iUM, 
what  to  Meteorology  ?  Which  presents  l^  largest  n'elu  1 


NATURAL  PHILOSOPHY. 

Tided,  compared  with  its  ancient  signification.     The 
complete  investigation  of  almost  any  subject,  either  of 
nature  or  art,  usually,  in  fact,  enters  the  peculiar  pro- 
vince of  several  kindred  departments  of  science.     For 
example,  let  us  follow  so  simple  a  substance  as  bread, 
from  the  sowing  of  the   grain  to  its  consumption  as 
food,  and  we  shall  find  that  the  successive  processes 
involve,  alternately,  the  principles  of  Mechanical  Phi- 
losophy, Chemistry,  and  Physiology.     The  ploughing 
of  the  field  is  mechanical  and  not  chemical,  because 
it  acts  on  masses  of  matter,  and  produces  no  change 
of  nature  in  the  matter  on  which  it  operates,  so  as  to 
make  it  something  different  from  what  it  was  before, 
but  merely  changes  its  place.     For  similar  reasons  the 
sowing  of  the  grain  is  mechanical.     But  now  a  change 
occurs  in  the    nature  of  the  seed.     By  the   process 
called  germination,  it  sprouts  and  grows  and  becomes 
a  living  plant.     As  this  is  a  change  which  takes  place 
between  the  particles  of  matter,  and  changes  the  na- 
ture of  the  body,  it  seems,  by  our  definition,  to  belong 
to  Chemistry,  and  it  would  do  so  were  not  the  changes 
those  of  living  matter  :  that  brings  it  under  the  head 
of  Physiology.     All   that   relates  to  the  growth   and 
perfecting  of  the  crop  is,  in  like  manner,  physiological. 
The  reaping,  carting,  and  threshing  the  wheat,  are  all 
mechanical  processes,  acting  as  they  do  on  masses  of 
matter,   and    producing   no  alteration    of  nature,  but 
merely  a  change  of  place.     The  grinding  and  separa- 
tion of  the  grain  into  flour  and  bran,  looks  like  a  chem- 
ical process,  because  it  reduces  the  wheat  to  particles, 
and  brings  out  two  new  substances.     We  have,  how- 
ever, only  changed  the  figure  and  place.     The  grain 

10.  What  is  strictly  our  subject  7  What  other  name  does  it  still  re- 
tain 1  What  is  true  of  the  complete  investigation  of  any  subject  in 
nature  or  art  1  How  exemplified  in  the  case  of  bread  ?  Why  is  the 
ploughing  mechanical  1  Why  is  the  sowing  mechanical  1  Why  is  the 
germination  physiological  1  How  is  it  with  the  reaping,  carting,  and 
threshing  1  The  grinding  and  manufacture  into  flour  ?  Making  the 


INTRODUCTION.  17 

consists  of  the  same  particles  before  and  after  grind- 
ing, and  no  new  substance  is  really  produced  by  the 
separation  of  the  flour  from  the  bran,  for  both  were  con- 
tained in  the  mixture,  having  the  same  nature  before  as 
after  the  separation.  We  next  mix  together  flour,  water, 
and  yeast,  to  make  bread,  and  bring  it  to  the  state  of 
dough.  So  far  the  process  is  mechanical  ;  but  now 
the  particles  of  these  different  substances  begin  to  act 
on  each  other,  by  the  process  called  fermentation,  and 
new  substances  are  produced,  not  existing  before  in 
either  of  the  ingredients,  and  the  whole  mass  becomes 
something  of  a  very  different  nature  from  either  of  the 
articles  of  which  it  was  formed.  Here  then  is  a  chem- 
ical change.  Next  we  make  the  dough  into  loaves  and 
place  them  in  the  oven  by  processes  which  are  me- 
chanical ;  but  again  heat  produces  new  changes  among 
the  particles,  and  brings  out  a  new  substance,  bread, 
which  is  entirely  different  in  its  nature  both  from  the 
original  'ingredients  and  from  dough.  This  change, 
therefore,  is  chemical.  Finally,  the  bread  is  taken  into 
the  mouth,  masticated,  and  conveyed  to  the  stomach 
by  mechanical  operations  ;  but  here  it  is  subjected  to 
the.  action  of  the  principle  of  life  that  governs  the  ani- 
mal system,  and  therefore  again  comes  under  the  pro- 
vince of  physiology. 

11.  The  distinction  between  terms,  which  are  apt 
to  be  confounded  with  each  other,  may  frequently  be 
expressed  by  single  words  or  short  phrases,  although 
they  may  not  convey  full  and  precise  definitions.  The 
following  are  examples  :  History  respects  facts  ;  Phi- 
losophy,  causes  ;  Physics,  matter  ;  Metaphysics,  mind ; 
Science,  general  principles ;  Art,  rules  and  instruments. 
Physical  laws  are  modes  of  action  ;  moral  and  civil 

bread  ?  Its  fermentation  1  Forming  into  loaves  and  p  lacing  in  tha 
oven  ?  The  baking  ? — eating  1 — the  final  change  in  the  stomach  1 

11.  What  does  History  respect  1  What  Philosophy  '{—Physics  ?— 
Metaphysics  r{ — Science? — Art  1  What  are  physical  and  what 

moral  laws  1    What  is  the  province  of  Natural,  and  -what  that  of 
<-k* 


18  NATURAL  PHILOSOPHY. 

laws,  rules  of  action.  The  province  of  Natural  Philos- 
ophy is  the  material  world  ;  that  of  Moral  Philosophy 
is  the  soul.  Mechanical  effects  result  from  change  of 
place  or  figure  ;  Chemical,  from  change  of  nature. 
Chemical  changes  respect  inanimate  matter ;  Physio- 
logical, living  matter. 

12.  Mechanical  Philosophy  takes  account  of  such 
properties  of  matter  only  as  belong  to  all  bodies  what- 
soever, or  of  such  as  belong  to  all  bodies  in  the  same 
state  of  solid,  fluid,  or  aeriform.  These  are  few  in 
number  compared  with  the  peculiar  properties  of  indi- 
vidual bodies,  and  the  changes  of  nature  which  they 
produce  on  each  other,  all  of  which  belong  to  Chemis- 
try. Chemistry,  therefore,  is  chiefly  occupied  with 
matter  ;  Natural  Philosophy,  with  motion.  The  lead- 
ing subjects  of  Natural  Philosophy  are — 

1.  MATTER — its  general  properties. 

2.  MECHANICS — the  doctrine  of  Motion. 

3.  HYDROSTATICS — the  doctrine  of  Fluids  in  the  form 
of  water. 

4.  PNEUMATICS — the  doctrine  of  Fluids  in  the  form 
of  air. 

5.  METEOROLOGY — the    Atmosphere   and   its    phe- 
nomena. 

6.  ACOUSTICS — the  doctrine  of  Sound. 

7.  ELECTRICITY. 

8.  MAGNETISM. 

9.  OPTICS — the  doctrine  of  Light. 

Moral  Philosophy!  From  what  do  mechanical  effects  result! — 
from  what  chemical  1  What  do  chemical  changes  respect,  and 
what  physiological !  _ 

12.  Of  wh  at  properties  does  Mechanical  Philosophy  take  account  1 
With  what  is  chemistry  chiefly  occupied  !— witn  what  is  Natural 
Philosophy  1  Enumerate  the  leading  subjects  of  Natural  Phi- 
iOsophy. 


CHAPTER  I. 
GENERAL  PROPERTIES  OF  MATTER. 

EXTENSION  AND    IMPENETRABILITY DIVISIBILITY — POROSITY — COM- 

PRESSIBILITY ELASTICITY INDESTRUCTIBILITY ATTRACTION. 

13.  All  matter  has  at  least  two  properties — Exten- 
sion and  Impenetrability.  The  smallest  conceivable 
portion  of  matter  occupies  some  portion  of  space,  and 
has  length,  breadth,  and  thickness.  Extension,  there- 
fore, belongs  to  all  matter.  Impenetrability  is  the 
property  by  which  a  portion  of  matter  excludes  all 
other  matter  from  the  space  which  it  occupies.  Thus, 
if  we  drop  a  bullet  into  water,  it  does  not  penetrate 
the  water,  it  displaces  it.  The  same  is  true  of  a  nail 
driven  into  wood.  These  two  properties  of  matter  are 
all  that  are  absolutely  essential  to  its  existence  ;  yet 
there  are  various  other  properties  which  belong  to 
matter  in  general,  or  at  least  to  numerous  classes  of 
bodies,  more  or  less  of  which  are  present  in  all  bodies 
with  which  we  are  acquainted.  Such  are  Divisibility, 
Porosity,  Compressibility,  Elasticity,  Indestructibility, 
and  Attraction.  Matter  exists  in  three  different  states, 
of  solids,  liquids,  and  gases.  These  result  from  its 
relation  to  heat ;  and  the  same  body  is  found  in  one  or 
the  other  of  these  states,  according  as  more  or  less 
heat  is  combined  with  it.  Thus,  if  we  combine  with 
a  mass  of  ice  a  certain  portion  of  heat,  it  passes  from 
the  solid  to  the  liquid  state,  forming  water  ;  and  if  we 
add  to  water  a  certain  other  portion  of  heat,  it  passes 
into  the  same  state  as  air,  and  becomes  steam.  Chem- 
istry makes  known  to  us  a  great  number  of  bodies  in 

13.  What  are  the  two  essential  properties  of  matter  1  Why  does 
extension  belong  to  all  matter  1  Define  impenetrability,  and  give 
an  example.  What  other  properties  belong  to  matter  1  In  what 
three  diflerent  states  does  matter  exist  1  How  exemplified  in  wa- 


20  NATURAL  PHILOSOPHY. 

the  aeriform  state,  called  gases,  arising  from  the  union 
of  heat  with  various  kinds  of  matter.  The  particles 
which  compose  water,  for  example,  are  of  two  kinds, 
oxygen  and  hydrogen,  each  of  which,  when  united 
with  heat,  forms  a  peculiar  kind  of  air  or  gas. 

14.  Matter  is  divisible  into  exceedingly  minute  parts. 
A    leaf  of  gold,  which  is  about  three  inches  square, 
weighs  only  about  the  fifth  part  of  a  grain,  and  is  only 
the   282,000th   part  of  an  inch    in   thickness.     Soap 
bubbles,  when  blown  so  thin  as  to  display  their  gaudy 
colors,  are  not  more  than  the  2,000,000th  of  an  inch 
thick  ;  yet  every  such  film  consists  of  a  vast  number 
of  particles.     The    ultimate    particles   of  matter,    or 
those   which  admit  of  no  further  division,  are  called 
•atoms.     The  atoms  of  which  bodies  are  composed  are 
inconceivably  minute.     The  weight  of  an    atom    of 
lead  is  computed  at  less  than  the  three  hundred  bil- 
lionth part  of  a  grain.     Animalcules  (insects  so  small 
as  to  be  invisible  to  the  naked  eye,  and  seen  only  by 
the  microscope)  are  sometimes  so  small  that  it  would 
take  a  million  of  them  to  amount  in  bulk  to  a  grain  of 
sand  ;  yet  these  bodies  often  have  a  complete  organi- 
zation, like  that  of  the  largest  animals.     They  have 
numerous    muscles,    by    means   of  which    they   often 
move  with  astonishing  activity  ;  they  have  a  digestive 
system  by  which  their  nutriment  is  received  and  ap- 
plied to  every    part  of  their  bodies  ;    and  they  have 
numerous  vessels  in  which  the  animal  fluids  circulate. 
What  must  be  the  dimensions  of  a  particle  of  one  of 
these  fluids  [ 

15.  A  large  portion  of  the  volume  of  all  bodies  con- 
sists of  vacant  spaces,  or  pores.     Sponge,  for  example, 
exhibits  its  larger  pores  distinctly  to  the  naked  eye. 

ter  *?  What  are  bodies  in  the  state  of  air  called  1  What  agent 
maintains  matter  in  the  state  of  gas  1 

14.  Divisibility. — Examples  in  gold  leaf— soap  bubbles.  What  are 
atoms  1 — weight  of  an  atom  of  lead1?  What  are  animalcules  1i 
Show  the  extreme  minuteness  of  their  parts 


GENERAL  PROPERTIES  OF  MATTER.  21 

But  it  also  has  smaller  pores,  of  which  the  more  solid 
matter  of  the  sponge  itself  is  composed,  which  are 
usually  so  small  as  to  be  but  faintly  discernible  to  the 
naked  eye.  The  cells  which  these  parts  compose  are 
separated  by  a  thin  fibre,  which  itself  exhibits  to  the 
microscope  still  finer  pores  ;  so  that  we  find  in  the 
same  body  several  distinct  systems  of  pores.  Even 
the  heaviest  bodies,  as  gold,  have  pores,  since  water, 
when  enclosed  in  a  gold  ball  and  subjected  to  strong 
pressure,  may  be  forced  through  the  sides.  Most  an- 
imals and  vegetables  consist  in  a  great  degree  of  mat- 
ter that  is  exceedingly  porous,  leaving  abundant  room 
for  the  peculiar  fluids  of  each  to  circulate.  Thus,  a 
thin  slip  or  cross  section  of  the  root  or  small  limb  of  a 
tree,  exhibits  to  the  microscope  innumerable  cells  for 
the  circulation  of  the  sap. 

16.  All  bodies  are  more  or  less  compressible,  or  may 
be  reduced  by  pressure  into  a  smaller  space.     Bodies 
differ  greatly  in  respect  to  this  property.     Some,  as 
air  or  sponge,  may  be  reduced  to  a  very  small  part  of 
their  ordinary  bulk,  while  others,    as  gold  and  most 
kinds  of  stone,  yield  but  little  to  very  heavy  pressures. 
Still,  columns  of  the  hardest  granite  are  found  to  un- 
dergo a  perceptible  compression  when  they  are  made 
to   support   enormous   buildings.      Water   and    other 
liquids  strongly  resist  compression,  but  still  they  yield 
a  little  when  pressed  by  immense  forces. 

17.  Many  bodies,  after  being  compressed  or  extended, 
restore  themselves  to  their  former  dimensions,  and  hence 
are  called  elastic.     Air  confined  in  a  bladder,  a  sponge 
compressed  in  the  hand,  and  India-rubber  drawn  out, 
are  familiar  examples  of  elastic  bodies.     If  we  drop 

15.  Porosity  — Example   in  sponge.     What  proof  is  there  that 
gold  is  porous  1    How  do  we  learn  that  animal  and  vegetable  mat- 
ter is  porous  1 

16.  Compressibility. — How  do  bodies  differ  in  this  respect .  1  What 
bodies  easily  yield  to  pressure  1 — what  yield  little  1    How  is  it  with 
granite  1 — with  water  1 


22 


NATURAL  PHILOSOPHY. 


Fig.  1. 


on  the  floor  a  ball  of  yarn,  or  of  ivory  or  glass,  it  re- 
bounds, being  more  or  less  elastic  ;  whereas,  if  we  do 
the  same  with  a  ball  of  lead,  it  falls  dead  without  re- 
bounding, and  is  therefore  non-elastic.  When  a  body 
perfectly  recovers  its  original 
dimensions,  it  is  said  to  be 
perfectly  elastic.  Thus,  air  is 
perfectly  elastic,  because  it 
completely  recovers  its  former 
volume,  as  soon  as  the  corn- 
pressing  force  is  removed, 
"  and  hence  resists  compression 
with  a  force  equal  to  that 
which  presses  upon  it.  Wood, 
when  bent,  seeks  to  recover 
itself  on  account  of  its  elasti- 
city ;  and  hence  its  use  in  the 
bow  and  arrow,  the  force  with 
which  it  recovers  itself  being 
suddenly  imparted  to  the  ar- 
row through  the  medium  of 
the  string. 

18.  Matter  is  wholly  indestructible.  In  all  the  chan- 
ges which  we  see  going  on  in  bodies  around  us,  not  a 
particle  of  matter  is  lost ;  it  merely  changes  its  form  ; 
nor  is  there  any  reason  to  believe  that  there  is  now  a 
particle  of  matter  either  more  or  less  than  there  was 
at  the  creation  of  the  world.  When  we  boil  water 
and  it  passes  to  the  invisible  state  of  steam,  this,  on 
cooling,  returns  again  to  the  state  of  water,  without 
the  least  loss  ;  when  we  burn  wood,  the  solid  matter 
of  which  it  is  composed  passes  into  different  forms, 


17.  Elasticity. — Give  examples.     Show  the  difference  bet'veen 
balls  of  ivory  and  lead.     When  is  a  body  perfectly  elastic  1    Give 
an  example.     Explain  the  philosophy  of  t'he  bow  and  arrow. 

18.  Indestructibility. — Is  matter  ever  annihilated  or  destroyed  1 
What  becomes  of  water  when  boiled,  and  of  wood  when  burned  1 


GENERAL  PROPERTIES  OF  MATTER.  23 

some  into  smoke,  some  into  different  kinds  of  airs,  or 
gases,  some  into  steam,  and  some  remains  behind  in 
the  state  of  ashes.  If  we  should  collect  all  these 
various  products,  and  weigh  them,  we  should  find  the 
amount  of  their  several  weights  the  same  as  that  of 
the  body  from  which  they  were  produced,  so  that  no 
portion  is  lost.  Each  of  the  substances  into  which 
the  wood  was  resolved,  is  employed  in  the  economy 
of  nature  to  construct  other  bodies,  and  may  finally 
reappear  in  its  original  form.  In  the  same  manner, 
the  bodies  of  animals,  when  they  die,  decay  and  seem 
to  perish  ;  but  the  matter  of  which  they  are  composed 
merely  passes  into  new  forms  of  existence,  and  reap- 
pears in  the  structure  of  vegetables  or  other  animals. 
19.  All  matter  attracts  all  other  matter.  This  is 
true  of  all  bodies  in  the  Universe.  In  this  extensive 
sense,  attraction  is  called  Universal  Gravitation.  In 
consequence  of  the  attraction  of  the  earth  for  bodies 
near  it,  they  fall  toward  it,  arid  this  kind  of  attraction 
is  called  Gravity.  Several  distinct  cases  of  this  prop- 
erty  occur  also  among  the  particles  of  matter.  That 
which  unites  particles  of  the  same  kind  (as  those  of  a 
musket  ball)  in  one  mass,  is  called  Aggregation  ;  fliat 
which -unites  particles  of  different  kinds,  forming  a 
compound,  (as  the  particles  of  flour,  water,  and  yeast 
in  bread,)  is  Affinity.  The  term  Cohesion  is  used  to 
denote  simply  the  union  of  the  separate  parts  that 
make  up  a  mass,  without  considering  whether  the  par- 
ticles themselves  are  simple  or  compound.  Thus  the 
grains  which  form  a  rock  of  sandstone,  are  united  by 
cohesion.  Magnetism  and  electricity  also  severally 
endue  different  portions  of  matter  with  tendencies 
either  to  attract  or  repel  each  other,  which  are  called, 

What  becomes  of  the  bodies  of  animals  when  they  die  1 
19.    Attraction. — How   extensive  1    What  is  it  called  when  ap- 
plied to  all  the  bodies  in  the  universe  1    Why  do  bodies  fall  toward 
the  earth  1  What  is  this  kind  of  attraction  called  1  What  is  aggrega- 
tion 1 — affinity  1 — cohesion  1    Give  an  example  of  each.    Wnai  are 


24  NATURAL  PHILOSOPHY. 

respectively,  Magnetic  and  Electric  attractions.  Te- 
nacity, or  that  force  by  which  the  particles  of  matter 
hang  together,  is  only  a  form  of  cohesion.  Of  all 
known  substances,  iron  wire  has  the  greatest  tenacity. 
A  number  of  fine  wires  bound  together  constitute  what 
is  called  a  wire  cable.  These  cables  are  of  such  pro- 
digious strength  that  immense  bridges  are  sometimes 
Fig.  2. 


suspended  by  them.     The  Menai  bridge,  in  Wales, 

one  of  the  greatest  works  in  modern  times,    is  thus 

supported  at  a  great  height,  although  it  weighs  toward 
two  thousand  tons. 


CHAPTER   II. 
MECHANICS. 

MOTION    IN    GENERAL LAWS    OF     MOTION CENTER    OF   GRAVITY 

PRINCIPLES  OF  MACHINERY. 

20.  MECHANICS,  or  tfre  DOCTRINE  OF  MOTION,  is  that 
part  of  Natural  Philosophy  which  treats  of  the  laws  of 
equilibrium  and  motion.  It  considers  also  the  nature 
of  the  forces  which  put  bodies  in  motion,  or  which 
maintain  them  either  in  motion,  or  in  a  state  of  rest  or 
equilibrium.  The  great  principles  of  motion  are  the 

magnetic   and   electric  attractions  1    Define  tenacity.     What  sub- 
stance has  the  greatest  1    How  employed  in  bridges  1 
20.  Define  mechanics.    What  are  those  agents  called  which  pat 


MECHANICS.  25 

same  everywhere,  being  applicable  alike  to  solids, 
liquids,  and  gases  ;  to  the  most  common  objects  around 
us,  and  to  the  heavenly  bodies.  The  science  of  Me- 
chanics, therefore,  comprehends  all  that  relates  to  the 
laws  of  motion  ;  to  the  forces  by  which  motion  is  pro- 
duced and  maintained  ;  to  the  principles  and  construc- 
tion of  all  machines ;  and  to  the  revolutions  of  the 
heavenly  bodies. 

SECTION  1. — Of  Motion  in  general. 

21.  Motion  is  change  of  place  from  one  point  of 
space  to  another.  It  is  distinguished  into  real  and 
apparent ;  absolute  and  relative  ;  uniform  and  variable. 
In  real  motion,  the  moving  body  itself  actually  changes 
place ;  in  apparent  motion,  it  is  the  spectator  that 
changes  place,  but  being  unconscious  of  his  own  mo- 
tion, he  refers  it  to  objects  without  him.  Thus,  when 
we  are  riding  rapidly  by  a  row  of  trees,  these  seem 
to  move  in  the  opposite  direction  ;  the  shore  appears 
to  recede  from  the  sailor  as  he  rapidly  puts  to  sea  ; 
and  the  heavenly  bodies  have  an  apparent  daily  motion 
westward,  in  consequence  of  the  spectator's  turning 
with  the  earth  on  its  axis  to  the  east.  Absolute  mo- 
tion is  a  change  of  place  from  one  point  of  space  to 
another  without  reference  to  any  other  body  :  Relative 
motion  is  a  change  of  position  with  respect  to  some 
other  body.  Two  bodies  may  both  be  in  absolute  mo- 
tion, but  if  they  do  not  change  their  position  with 
respect  to  each  other,  they  will  have  no  relative  mo- 
tion, or  will  be  relatively  at  rest.  The  men  on  board 
a  ship  under  sail,  have  all  the  same  absolute  motion, 

bodies  in  motion  or  keep  them  at  rest  1  How  extensively  do  the 
great  principles  of  motion  prevail  ]  What  does  the  science  of  me- 
chanics comprehend  1 

21.  Define  motion.    Into  what  varieties  is  it  distinguished  1    Ex- 
plain the  difference  between  real  and  apparent  motion.     Give  ex- 
amples of  apparent  motion.    Distinguish  between  absolute  and  re- 
lative motion.    Example  in  the  case  of  persons  on  board  a  ship — 
3 


26  NATURAL  PHILOSOPHY. 

and  so  long  as  they  are  still,  they  have  no  other  ;  but 
whatever  changes  of  place  occur  among  themselves, 
give  rise  to  relative  motions.  If  two  persons  are 
travelling  the  same  way,  at  the  same  rate,  whether  in 
company  or  not,  they  have  no  relative  motion  ;  if  one 
goes  faster  than  the  other,  the  latter  has  a  relative 
motion  backward  equal  to  the  difference  of  their  rates  ; 
and  if  they  are  travelling  in  opposite  directions,  their 
relative  motion  is  equal  to  the  sum  of  both  their  mo- 
tions. A  body  moves  with  a  uniform  motion  when  it 
passes  over  equal  spaces  in  equal  times  ;  with  a  vari- 
able motion,  when  it  passes  over  unequal  spaces  in 
equal  times.  If  a  man  walks  over  just  as  many  feet 
of  ground  the  second  minute  as  the  first,  and  the  third 
as  the  second,  his  motion  is  uniform  ;  but  if  he  should 
walk  thirty  feet  one  minute,  lorty  the  next,  and  fifty 
the  next,  his  motion  would  be  variable. 

22.  Force  is  any  thing  that  moves,  o,r  lends  to  move  a 
body.  The  strength  of  an  animal  exerted  to  draw  a 
carriage,  the  impulse  of  a  waterfall  in  turning  a  wheel, 
and  the  power  of  steam  in  moving  a  steamboat,  are  sev- 
erally examples  of  a  force.  A  weight  on  one  arm  of 
a  pair  of  steelyards,  in  equilibrium  with  a  piece  of 
merchandise,  although  it  does  not  move,  but  only  tends, 
to  move  the  body,  is  still  a  force,  since  it  would  pro- 
duce motion  were  it  not  counteracted  by  an  equal  force. 
The  quantity  of  motion  in  a  body  is  called  its  momen- 
tum. Two  bodies  of  equal  weight,  as  two  cannon- 
balls,  will  evidently  have  twice  as  much  motion  as 
one  ;  nor  would  it  make  any  difference  if  they  were- 
united  in  one  mass,  so  as  to  form  a  single  body  of 
twice  the  weight  of  the  separate  balls  ;  the  quantity  of 
motion  would  be  doubled  by  doubling  the  mass,  while 
the  velocity  remained  the  same.  Again,  a  ball  that 

in  the  case  of  travellers  1    When  does  a  body  move  with  uniform 
jnotion  1    When  with  variable  motion  *?    Example. 
22.  Define  force.    £&amples.  Wha,t  is  momentum  1    Upon  what 


r  MECHANICS.  27 

moves  twice  as  fast  as  before,  has  twice  the  quantity 
of  motion.  Momentum  therefore  depends  upon  two 
things — the  velocity  and  quantity  of  matter.  A  large 
body,  as  a  ship,  may  have  great  momentum  with  a  slow 
motion ;  a  small  body,  as  a  cannon-ball,  may  have 
great  momentum  with  a  swift  motion  ;  but  where  great 
quantity  of  matter  (or  mass)  is  united  with  great  swift- 
ness, the  momentum  is  greatest  of  all.  Thus  a  train 
of  cars  on  a  railroad  moves  with  prodigious  momen- 
tum ;  but  the  planets  in  their  revolutions  around  the  sun, 
with  a  momentum  inconceivably  greater. 

23.  To  the  eye  of  contemplation,  the  world  presents 
a  scene  of  boundless  activity.  On  the  surface  of  the 
earth,  hardly  any  thing  is  quiescent.  Every  tree  is 
waving,  and  every  leaf  trembling  ;  the  rivers  are  run- 
ning to  the  sea,  and  the  ocean  itself  is  in  a  state  of 
ceaseless  agitation.  The  innumerable  tribes  of  ani- 
mals are  in  almost  constant  motion,  from  the  minutest 
insect  to  the  largest  quadruped.  Amid  the  particles  of 
matter,  motions  are  unceasingly  going  forward,  in  as- 
tonishing variety,  that  are  effecting  all  the  chemical 
and  physiological  changes  to  which  matter  is  constantly 
subjected.  And  if  we  contemplate  the  same  subject 
on  a  larger  scale,  we  see  the  earth  itself,  and  all  that 
it  contains,  turning  with  a  steady  and  never  ceasing 
motion  around  its  own  axis,  wheeling  also  at  a  vastly 
swifter  rate  around  the  sun,  and  possibly  accompany- 
ing the  sun  himself  in  a  still  grander  circuit  around 
some  distant  center.  Hence,  almost  all  the  phenom- 
ena or  effects  which  Natural  Philosophy  has  to  inves- 
tigate and  explain  are  connected  with  motion  and  de- 
pendent on  it. 

two  things  does  it  depend  *?  What  union  of  circumstances  produces 
great  momentum  1  Example. 

23.  What  proofs  of  activity  do  we  see  in  nature  1  Give  examples 
in  the  vegetable  kingdom — in  the  animal— among  the  particles  of 
matter — and  among  the  heavenly  bodies.  Upon  what  are  almost 
all  the  phenomena  of  Natural  Philosophy  dependent  1 


28  NATURAL  PHILOSOPHY. 

SEC.  2. — Of  the  Laws  of  Motion. 

24.  Nearly  all  the  varieties  of  motion  that  fall  with 
in  the  province  of  Mechanical  Philosophy,  have  been 
reduced  to  three  great  principles,  called  the  Laws  of 
Motion.  We  will  consider  them  separately. 

FIRST  LAW. — Every  body  will  persevere  in  a  state  of 
rest,  or  of  uniform  motion  in  a  straight  line,  until  com- 
pelled by  some  force  to  change  its  state.  This  law 
contains  four  separate  propositions  ;  first,  that  unless 
put  in  motion  by  some  external  force,  a  body  always 
remains  at  rest ;  secondly,  that  when  once  in  motion 
it  always  continues  so  unless  stopped  by  some  force  ; 
thirdly,  that  this  motion  is  uniform  ;  and  fourthly,  that 
it  is  in  a  straight  line.  Thus,  if  I  place  a  ball  on  a 
smooth  sheet  of  ice,  it  will  remain  constantly  at  rest 
until  some  external  force  is  applied,  having  no  power 
to  move  itself.  I  now  apply  such  force  and  roll  it ; 
being  set  in  motion,  it  would  move  on  forever  were 
there  no  impediments  in  the  way.  It  will  move  uni- 
formly, passing  over  equal  spaces  in  equal  times,  and 
it  will  move  directly  forward  in  a  straight  course,  turn- 
ing neither  to  the  right  hand  nor  to  the  left.  This 
property  of  matter  to  remain  at  rest  unless  something 
moves  it,  and  to  continue  in  motion  unless  something 
stops  it,  is  called  Inertia.  Thus  the  inertia  of  a  steam- 
boat opposes  great  resistance  to  its  getting  fully  into 
motion  ;  but  having  once  acquired  its  velocity,  it  con- 
tinues by  its  inertia  to  move  onward  after  the  engine  is 
stopped,  until  the  resistance  of  the  water  and  other 
impediments  destroy  its  motion.  The  planets  continue 
to  revolve  around  the  sun  for  no  other  reason  than 
this,  that  they  were  put  in  motion  and  meet  with  noth- 
ing to  stop  them.  Whenever  a  horse  harnessed  to  a 
carriage  starts  suddenly  forward,  he  breaks  his  traces, 

24.  To  how  many  great  principles  have  all  the  varieties  of  motion 
been  reduced  1  What  are  they  called  1  State  the  first  law.  Enu- 
merate the  four  propositions  contained  in  this  law.  Example. 


MECHANICS. 


29 


because  the  inertia  of  the  carriage  prevents  the  sudden 
motion  being  instantly  propagated  through  its  mass, 
and  the  force  of  the  horse  being  all  expended  on  the 
traces,  breaks  them.  On  the  other  hand,  if  a  horse 
suddenly  stops,  when  on  a  run,  the  rider  is  thrown 
over  his  head  ;  for  having  aco1uired  the  full  motion  of 
the  horse,  he  does  not  instantly  lose  it,  but,  on  ac- 
count of  his  inertia,  continues  to  move  forward  after 
the  force  that  put  him  in  motion  is  withdrawn.  This 


Fig.  3. 


principle  is  pleasingly  illus- 
trated in  what  is  called  the 
doubling  of  the  hare.  A  hare 
closely  pursued  by  a  grey- 
hound, starts  from  A,  and  when 
he  arrives  at  C,  the  dog  is  - 
hard  upon  him  ;  but  the  hare 
being  a  lighter  animal  than 
the  dog,  and  having  of  course 
less  inertia,  turns  short  at  C 
and  again  at  E,  while  the  dog 
cannot  stop  so  suddenly,  but  goes  further  round  at 
D  and  also  F,  and  thus  the  hair  outruns  him.  Put 
a  card  of  pasteboard  across  a  couple  of  wine  glass- 
es, and  two  sixpences  di- 
rectly over  the  glasses, 
as  in  the  figure ;  then 
strike  the  edge  of  the 
card  at  A  a  smart  blow, 
and  the  card  will  slip 
off  and  leave  the  money 
in  the  glasses.  The 
coins,  on  account  of  their  inertia,  do  not  instantly 
receive  the  motion  communicated  to  the  card.  If  the 
blow,  however,  be  gentle,  all  will  go  off  together. 


Fig.  4. 


"What  is  inertia  1    Example  in  a  steamboat — in  the  planets — in  a 
horse — in  the  doubling  of  a  hare — and  in  the  card  and  com. 


30 


NATURAL  PHILOSOPHY. 


Fig.  5. 


.iiiiirniiinnmii 


_____ 


25.  The  first  law  of  motion  also  asserts,  that  all 
moving  bodies  have  a  tendency  to  move  in  straight 
lines.  We  see,  indeed,  but  few  examples  of  such 
motions  either  in  nature  or  art.  If  we  throw  a  ball 
upward,  it  rises  and  falls  in  a  curve  ;  water  spouting 
into  the  air  does  the  same  ;  rivers  usually  run  and 
trees  wave  in  curves  ;  and  the  heavenly  bodies  re- 
volve in  apparent  circles.  Still,  when  we  attentively 
examine  each  of  these  cases,  and  every  other  case  of 
motion  in  curves,  we  find  one  or  more  forces  opera- 
ting to  cause  the  body  to  deviate  from  a 
straight  line.  When  such  cause  of  de- 
.  viation  is  removed,  the  body  immedi- 
ately resumes  its  progress  in  a  straight 
line.  This  effort  of  bodies,  when  mov- 
ing in  curves,  to  proceed  directly  for- 
ward in  a  straight  line,  is  called  the 
Centrifugal  Force.  If  we  turn  a  grind- 
stone, the  lower  part  of  which  dips  into 
water,  as  the  velocity  increases  the 
water  is  thrown  off  from  the  rim  in 
straight  lines  which  touch  the  rim  and 
are  therefore  called  tangents*  to  it ;  and 
it  is  a  general  principle,  that  when  bodies 
free  to  move,  revolve  in  curves  about  a 
center,  they  have  a  constant  tendency  to 
fly  off  in  straight  lines,  which  are  tan- 
|c  gents  to  the  curves.  We  see  this  princi- 
ple exemplified  in  giving  a  rotary  mo- 
tion to  a  pail  or  basin  of  water.  The 
liquid  first  rises  on  the  sides  of  the  vessel,  and  if  the 
rapidity  of  revolution  be  increased,  it  escapes  from 

*  A  line  is  said  to  be  a  tangent  to  a  curve,  when  it  touches  the  curve,  but 
does  not  cut  it. 

25.  Are  the  motions  observed  in  the  natural  world,  usually  per- 
formed in  straight  or  in  curved  lines  1  Why  then  is  it  said  that  bod- 
ies naturally  move  in  straight  lines  1  What  is  this  effort  to  move  in 


MECHANICS.  31 

the  top  in  straight  lines  which  are  tangents  to  the  rim 
of  the  vessel.  If  we  pass  a  cord  through  a  staple  in 
the  ceiling  of  a  room,  and  bringing  down  the  two 
ends,  attach  them  to  the  ears  of  a  pail  containing  a 
little  water,  (suspending  the  vessel  a  few  feet  above 
the  floor,)  and  then,  applying  the  palms  of  the  hands 
to  the  opposite  sides  of  the  pail,  give  it  a  steady  rotary 
motion,  the  water  will  first  rise  on  the  sides  of  the 
vessel  and  finally  be  projected  from  the  rim  in  tan- 
gents.  The  experiment  is  more  striking  if  we  suffer 
the  cord  to  untwist  itself  freely,  after  having  been 
twisted  in  the  preceding  process. 

26.  SECOND  LAW.  Motion,  or  change  of  motion, 
is  proportioned  to  the  force  impressed,  and  is  produced 
in  the  line  of  direction  in  which  that  force  acts.  First, 
the  quantity  of  motion,  or  momentum,  is  proportioned 
to  the  force  applied.  A  double  blow  produces  a 
double  velocity  upon  a  given  mass,  or  the  same  velo- 
city upon  twice  the  mass.  Two  horses  applied  with 
equal  advantage  to  a  load,  will  draw  twice  the  load 
of  one  horse.  It  follows  also  from  this  law,  that  every 
force  applied  to  a  body,  however  small  that  force  may 
be,  produces  some  motion.  A  stone  falling  on  the 
earth  moves  it.  This  may  seem  incredible  ;  but  if 
we  suppose  the  earth  divided  into  exceedingly  small 
parts,  each  weighing  only  a  pound  for  example,  then 
we  may  readily  conceive  how  the  falling  stone  would 
put  it  in  motion.  Now  the  effect  is  not  lost  by  being 
expended  on  the  whole  earth  at  once  ;  the  momen- 
tum produced  is  the  same  in  both  cases  ;  but  in  pro- 
portion as  the  quantity  of  matter  is  increased  the 
velocity  is  diminished,  and  it  would  be  as  much  less 


straight  lines  called  1    Example  in  a  grindstone — in  a  suspended 
vessel  of  water. 

26.  What  is  the  second  law  of  motion  1  Show  that  the  quantity 
of  motion  is  proportioned  to  the  force  applied.  Explain  now  the 
smallest  force  produces  some  motion. 


32  NATURAL  PHILOSOPHY. 

as  the  weight  of  the  whole  earth  exceeds  one  pound. 
It  would  therefore  be  inappreciable  to  the  senses,  but 
still  capable  of  being  expressed  by  a  fraction,  and 
therefore  a  real  quantity.  "  A  continual  dropping 
wears  away  stone."  Each  drop,  therefore,  must  con- 
tribute something  to  the  effect,  although  too  small  to 
be  perceived  by  itself. 

27.  Secondly,  motion  is  produced  in  the  line  of 
direction  in  which  the  force  is  applied.  If  I  lay  a 
ball  on  the  table  and  snap  it  with  my  thumb  and  finger, 
it  moves  in  different  directions  according  as  I  change 
the  direction  of  the  impulse  ;  and  this  is  conformable 
to  all  experience.  A  single  force  moves  a  body  in 
its  own  direction,  but  two  forces  acting  on  a  body  at 
the  same  time,  move  it  in  a  line  that  is  intermediate 
between  the  two.  Thus,  if  I  place  a  small  ball,  as  a 
marble,  on  the  table,  and  at  the  same  moment  snap  it 
with  the  thumb  and  finger  of  each  hand,  it  will  not 
move  in  the  direction  of  either  impulse,  but  in  a  line 
between  the  two.  A  more  precise  consideration  of 
this  case  has  led  to  the  following  important  law  : 

If  a  body  is  impelled  by  two  forces  which  may  be  re- 
presented in  quantity  and  direction  by  the  two  sides  of 
a  parallelogram,  it  will  describe,  the  diagonal  in  the  same 
time  in  which  it  would  have  described  each  of  the  sides 
separately,  by  the  force  acting  parallel  to  that  side. 

Thus,  suppose  the  parallelogram  A  B  C  D,  repre- 
sents a  table,  of  which  the  side  A  B  is  just  twice  the 
length  of  A  D.  I  now  place  the  ball  on  the  corner  A, 
and  nail  a  steel  spring  (like  a  piece  of  watch  spring) 
to  each  side  of  the  corner,  so  that  when  bent  back  it 
may  be  sprung  upon  the  ball,  and  move  it  parallel  to 
the  edge  of  the  table.  I  first  spring  each  force  sep- 
arately, bending  back  that  which  acts  parallel  to  the 

27.  Show  that  motion  is  in  the  tine  of  direction  of  the  force. 
How  does  a  single  force  move  a  body  1  How  do  two  forces  move 
if?  Ilecite  the  law  represented  in  figured,  and  explain  the  figure. 


MECHANICS. 


33 


longer  side  so  much  further  than  the  other,  that  the 
ball  will  move  over  the  two  sides  in  precisely  the 
same  time,  sup-  j^g.  g 

pose  two  sec-  Jy 
onds.  I  now  let 
off  the  springs 
on  the  ball  at 
the  same  in- 
stant, and  the 
ball  moves  a- 
cross  the  table,? 
from  corner  to 
corner,  in  the 
same  two  sec- 
onds. It  is  not  necessary  that  the  parallelogram 
should  be  right-angled'  like  a  table.  The  effect  will 
be  the  same  at  whatever  angle  the  sides  of  the  paral- 
lelogram meet. 

28.  If  I  take  a  triangular  board  instead  of  the  table, 
and  fix  three  springs  at  one  corner,  so  as  to  act  paral- 
lel to  the  three  sides  of  the  board,  and  give  each 
spring  a  degree  of  strength  proportioned  to  the  length 
of  the  side  in  the  direction  of  which  it  acts,  and  then 
let  all  those  springs  fall  upon  the  ball  at  the  same  in- 
stant, the  ball  will  remain  at  rest.  This  fact  is  ex- 
pressed in  the  following  proposition  : 

If  three  forces,  represented  in  quantity  and  direction 
by  the  three  sides  of  a  triangle,  act  upon  a  body  at  the 
same  time,  it  will  be  kept  at  rest. 

A  kite  is  seen  to  rest  in  the  air  on  this  principle, 
being  in  equilibrium  between  the  force  of  gravity 
which  would  carry  it  toward  the  earth,  that  of  the 
string,  and  that  of  the  wind,  which  severally  act  in 
the  three  directions  of  the  sides  of  a  triangle,  and 

28.  What  is  the  effect  of  three  forces,  represented  in  quantity  and 
direction  by  the  three  sides  of  a  triangle  1  How  does  a  kite  exem- 
plify this  principle  1  Is  the  principle  confined  to  three  directions  1 


34 


NATURAL  PHILOSOPHY. 


B  Fig.  7. 


neutralize  each  other.  Nor  is  the  principle  confined 
to  three  directions  merely,  but  holds  good  for  a  poly- 
gon of  any  number  of  sides.  For  example,  a  body 
situated  at  A,  and  acted  upon  by  five  forces  repre- 
sented in  quantity 
and  direction  by 
the  five  sides  of 
the  polygon,  (Fig. 
7,)  would  remain 
at  rest.  If  the  for- 
ces were  only  four, 
corresponding  to 
all  the  sides  of  the 
figure  except  the 
last,  EA,  then  the 
body  would  de- 
scribe this  side  in 
the  same  time  in 
which  it  would  de- 
forces acting  sepa- 


E 

of  the  sides 


by  the 


scribe   each 
rately. 

29.  Simple  motion  is  that  produced  by  one  force  ; 
compound  motion,  that  produced  by  the  joint  action  of 
several  forces.  Strictly  speaking,  we  never  witness 
an  example  of  simple  motion  ;  for  when  a  ball  is 
struck  by  a  single  impulse,  although  the  motion  is 
simple  relatively  to  surrounding  bodies,  yet  the  ball 
is  at  the  same  time  revolving  with  the  earth  on  its' 
axis  and  around  the  sun,  and  subject  perhaps  to  innu-- 
merable  other  motions.  Although  all  bodies  on  the 
earth  are  acted  on  at  the  same  moment  by  many  for- 
ces, and  therefore  it  is  difficult  or  even  impossible  to* 
tell  what  is  the  line  each  describes  in  space  under 

Case  of  a  polygon  of  five  sides.     "Where  only  four  forces  are  ap- 
plied, how  will  the  body  move  7 

29.  What  is  simple,  and  what  compound  motion  *?  Do  we  ever 
witness  simple  motions  in  nature  1  Example.  When  a  force  is 


MECHANICS.  35 

their  joint  action,  yet  each  individual  force  produces 
precisely  the  same  change  of  direction  in  the  body 
as  though  it  were  to  act  alone.  If  it  acts  in  the  same 
direction  in  which  the  body  is  moving,  it  will  add  its 
own  amount ;  if  in  the  opposite  direction,  it  will  sub- 
tract it ;  if  sidewise,  it  will  turn  the  body  just  as  far 
to  the  right  or  left  in  a  given  time,  as  it  would  have 
done  had  it  been  applied  to  the  body  at  rest.  Thus,  if 
while  a  body  is  moving  Fi  g 

from  A  to  B,  (Fig.  8,)  it  c  D 

be  struck  by  a  force  in  the 
direction  of  AC,  it  will 
reach  the  line  CD,  in  the 
same  time  in  which  it 
would  have  done  had  it 
been  subject  to  no  other 
force.  It  will,  however, 
reach  that  line  in  the  point 


D  instead  of  C.     When  a  A  B 

man  walks  the  decks  of  a  ship  under  sail,  his  motions 
are  precisely  the  same  with  respect  to  the  other  objects 
on  board,  as  though  the  ship  were  at  rest ;  but  the  line 
which  he  actually  describes  under  the  two  forces  is 
very  different. 

30.  Instances  of  this  diagonal  motion  are  con- 
stantly presented  to  our  notice.  In  crossing  a  river, 
the  boat  moves  under  the  united  impulses  of  the  oars 
and  the  current,  and  describes  the  diagonal  whose 
sides  are  proportional  to  the  two  forces  respectively. 
Equestrians  sometimes  exhibit  feats  of  horsemanship 
by  leaping  upward  from  the  horse  while  running,  and 
recovering  their  position  again.  They  have,  in  fact, 


applied  to  a  body  in  motion,  what  is  the  effect  1   Explain  from  Fi» 
8.     Case  of  a  man  walking  the  deck  of  a  ship, 

30.  Examples  of  diagonal  motion.     A   boat   crossing  a 
EquefctriansT— Two  men  ift  a  boat  togsing  $  ball^-i 


36  NATURAL  PHILOSOPHY. 

only  to  rise  and  fall  as  they  would  do  were  the  horse 
at  rest ;  for  the  forward  motion  which  the  rider  re- 
tains by  his  inertia,  during  the  short  interval  of  his 
ascent  and  descent,  carries  him  onward,  so  that  he 
rises  in  one  diagonal  and  falls  in  another.  Two  men 
sitting  on  opposite  sides  of  a  boat  in  rapid  motion,  will 
toss  a  ball  to  each  other  in  the  same  manner  as  though 
the  boat  were  at  rest ;  but  the  actual  movement  of  the 
ball  will  be  diagonal.  Rowing,  itself,  exemplifies  the 
same  principle  ;  for  while  one  oar  would  turn  the 
boat  to  the  left  and  the  other  to  the  right,  it  actually 
moves  ahead  in  the  diagonal  between  the  two  direc- 
tions. 

31.  When,  of  two  motions  impressed  upon  a  body, 
one  is  the  uniform  motion  which  results  from  an  im- 
pulse, and  the  other  is  produced  by  a  force  which  acts 
continually,   the   path   described   is    a  curve.     Thus, 
when  we  shoot  an  arrow  into  the  air,  the  impulse  given 
by  the  string  tends  to  carry  it  forward  uniformly  in  a 
straight  line  ;  but  gravity  draws  it  continually  away 
from  that  line,  and   makes  it  describe  a  curve.     In 
the  same  manner  the  planets  are  continually  drawn 
away  from  the  straight  lines  in  which  they  tend  to 
move,  by  the  attraction  of  the  sun,  and  are  made  to 
describe  curved  orbits  about  that  body. 

32.  THIRD   LAW.      When  bodies  act  on  each  other, 
action  and  reaction  are  equal,  and  in  opposite  directions. 
The  meaning  of  this  law  is,  that  when  a  body  imparts 
a  motion  in  any  direction,  it  loses  an  equal  quantity 
of  its  own — that  no  body  loses  motion  except  by  im- 
parting an  equal  amount  to  other  bodies — that  when  a 
body  receives  a  blow  it  gives  to  the  striking  body  an 
equal  blow — that  when  one  body  presses  on  another  it 
receives  from  it  an  equal  pressure — that  when  one  body 

31.  Under  what  two  forces  will  a  body  describe  a  curve  1  Exam- 
ples— An  arrow — The  planets. 

32.  Give  the  third  law  of  motion.    Explain  its  meaning.    Exam- 


MECHANICS.  37 

attracts  or  repels  another,  it.  is  equally  attracted  or  re- 
pelled by  the  other.  If  a  steamboat  should  run  upon 
a  sloop  sailing  in  the  same  direction  with  a  slower 
motion,  it  might  drive  it  headlong  without  experien- 
cing any  great  shock  itself;  still  its  own  loss  of  motion 
would  be  just  equal  to  that  which  it  imparted  to  the 
sloop,  but  being  distributed  over  a  quantity  of  matter 
so  much  greater,  the  loss  might  be  scarcely  percep- 
tible. If  a  light  body,  as  the  wad  of  a  cannon,  were 
fired  into  the  air,  it  would  be  stopped  by  the  resistance 
of  the  air  ;  but  its  own  motion  would  be  lost  only  as  it 
imparted  the  same  amount  to  the  air,  and  thus  might 
be  sufficient,  on  account  of  the  lightness  of  air,  to  set 
a  large  volume  in  motion.  When  the  boxer  strikes  his 
adversary,  he  receives  an  equal  blow  from  the  reaction 
of  the  part  struck  ;  but  receiving  it  on  a  part  of  less 
sensibility,  he  is  less  injured  by  it  than  his  adversary 
by  the  blow  inflicted  on  him.  One  who  falls  from  an 
eminence  on  a  bed  of  down,  receives  in  return  a  resist- 
ance equal  to  the  force  of  the  fall,  as  truly  as  one  who 
falls  on  a  solid  rock  ;  but,  on  account  of  the  elasticity 
of  the  bed,  the  resistance  is  received  gradually,  and  is 
therefore  distributed  more  uniformly  over  the  system. 
A  boatman  presses  against  the  shore,  the  reaction  of 
which  sends  the  boat  in  the  opposite  direction.  He 
strikes  the  water  with  his  oar  backward,  and  the 
boat  moves  forward.  The  fish  beats  the  water  with 
his  tail,  first  on  one  side  and  then  on  the  other,  and 
moves  forward  in  the  diagonal  between  the  two  reac- 
tions. The  bird  beats  the  air  with  her  wings,  and  the 
resistance  carries  her  forward  in  the  opposite  direc- 
tion. All  attractions  likewise  are  mutual.  The  iron 
attracts  the  magnet  just  as  much  as  the  magnet  attracts 
the  iron.  The  earth  attracts  the  sun  just  as  much  as 
the  sun  attracts  the  earth.  In  all  these  cases  the  mo- 

ptes  of  a  steamboat  running  upon  a  sloop — a  wad  fired  into  the  air^-a 

boxer— falling  upon  a  feather  bed— a  boatman— a  bird— attractions. 

4 


33 


NATURAL  PHILOSOPHY. 


Fig.  9. 


mentum  or  quantity  of  motion  in  the  smaller  and  the 
larger  body,  is  the  same.  Thus,  when  a  small  boat 
is  drawn  by  a  rope  toward  a  large  ship,  the  ship 
moves  toward  the  boat  as  well  as  the  boat  toward  the 
ship,  and  with  the  same  momentum  ;  but  the  space 
over  which  the  ship  moves  is  as  much  less  than  that 
of  the  boat,  as  its  quantity  of  matter  is  greater.  It 
makes  no  difference  whether  the  boat  is  drawn  to- 
ward the  ship  by  a  man  standing  in  the  boat  and  pull- 
ing at  a  rope  fastened  to  the  ship,  or  by  a  man  stand- 
ing in  the  ship  and  pulling  by  a  rope  fastened  to  the 
boat.  A  fisherman  once  fancied  he  could  manufacture 

a  breeze  for  himself 
by  mounting  a  pair 
of  huge  bellows  in 
the  stern  of  his  boat, 
and  directing  the 
blast  upon  the  sail. 
But  he  was  surprised 
to  find  that  it  had  no 
effect  on  the  motion 
of  the  boat.  We  see 
that  the  reaction  of 
the  blast  would  tend 
to  carry  the  boat 
backward  just  as  much  as  its  direct  action  tended  to 
carry  the  boat  forward. 

33.  FALLING  BODIES.  When  a  body  falls  freely 
toward  the  earth  from  some  point  above  it,  it  falls  con- 
tinually faster  and  faster  the  longer  it  is  in  falling.  Its 
motion  therefore  is  said  to  be  uniformly  accelerated. 
All  bodies,  moreover,  light  and  heavy,  would  fall 
equally  fast  were  it  not  for  the  resistance  of  the  air, 
which  buoys  up  the  lighter  body  more  than  it  does  the 

Compare  the  momentum  of  a  small  boat  with  that  of  a  large  ship  when 

drawn  together.  Case  of  a  man  who  put  a  pair  of  bellows  to  his  boat. 

33.  When  is  the  motion  of  a  body  said  to  be  uniformly  accelera- 


MECHANICS.  39 

heavier ;  but  in  a  space  free  from  air,  or  a  vacuum, 
a  feather  falls  just  as  fast  as  a  guinea.  If  a  boy  knocks 
a  ball  with  a  bat  on  smooth  ice,  it  will  move  on  uni- 
formly by  the  impulse  it  has  received  ;  but  if  several 
other  boys  strike  it  successively  the  same  way,  its 
velocity  is  continually  increased.  Now  gravity  is  a 
force  which  acts  incessantly  on  falling  bodies,  and 
therefore  constantly  increases  their  speed.  If  I  as- 
cend a  high  tower  and  let  a  ball  fall  from  my  hand 
to  the  ground,  it  will  fall  16T^  feet  in  one  second,  64£ 
in  two  seconds,  and  257i  in  four  seconds  ;  that  is,  a 
body  will  fall  four  times  as  far  in  two  seconds  as  in 
one,  and  sixteen  times  as  far  in  four  seconds  as  in 
one.  Now  four  is  the  square  of  two,  and  sixteen  is 
the  square  of  four  ;  so  that  the  spaces  described  by  a 
falling  body  are  proportioned,  not  simply  to  the  times 
of  falling,  but  to  the  squares  of  the  times  ;  so  that  a 
body  falls  in  ten  seconds  not  merely  ten  times  as  far 
as  in  one  second,  but  the  square  of  ten,  or  a  hundred 
times  as  far. 

34.  Hence,  when  bodies  fall  toward  the  earth  from 
a  great  height,  they  finally  acquire  prodigious  speed. 
A  man  falling  from  a  balloon  half  a  mile  high,  would 
reach  the  earth  in  about  half  a  minute.  We  seldom 
see  bodies  falling  from  a  great  height  perpendicularly 
to  the  earth  ;  but  even  in  rolling  down  inclined  planes, 
as  a  rock  descending  a  steep  mountain,  or  a  rail  car 
breaking  loose  from  the  summit  of  an  inclined  plane, 
we  see  strikingly  exemplified  the  nature  of  accele- 
rated motion.  A  log  descending  by  a  long  wooden 
trough  down  a  steep  hill,  has  been  known  to  acquire 
momentum  enough  to  cut  in  two  a  tree  of  considerable 

ted  1  How  would  a  guinea  and  a  feather  fall  in  a  vacuum  *?  Case  of 
a  ball  knocked  on  ice.  How  much  further  will  a  body  fall  in  two 
seconds  than  in  one  1  How  are  the  spaces  of  falling  bodies  pro- 
portioned to  the  times  of  falling  1 

34.-  In  what  time  would  a  man  fall  from  a  balloon  hah"  a  mile 
high  1  Where  do  we  see  the  rapid  acceleration  of  falling  bodies 


40  NATURAL  PHILOSOPHY. 

size,  which  it  met  on  leaping  from  the  trough.  At  a 
great  distance  from  the  earth,  the  force  of  gravity  be- 
comes sensibly  diminished,  so  that  if  we  could  ascend 
in  a  balloon  four  thousand  miles  above  the  earth,  that 
is,  twice  as  far  from  the  center  of  the  earth  as  it  is 
from  the  center  to  the  surface,  the  force  of  attraction 
would  be  only  one  fourth  of  what  it  is  at  the  surface  of 
the  earth,  and  a  body  instead  of  falling  16  feet  in  a 
second  would  fall  only  4  feet.  At  ten  times  the  distance 
of  the  radius  of  the  earth,  the  force  of  gravity  would 
be  only  one  hundredth  part  of  what  it  is  at  the  earth. 
This  fact  is  expressed  by  saying,  that  the  force  of 
gravity  is  inversely  as  the  square  of  the  distance  from 
the  center  of  the  earth,  diminishing  in  the  same  propor- 
tion as  the  square  of  the  distance  increases.  As  the 
moon  is  about  sixty  times  as  far  from  the  center  of  the 
earth  as  the  surface  of  the  earth  is  from  the  center,  if 
a  body  were  let  fall  to  the  earth  from  such  a  distance, 
(the  force  'of  gravity  being  the  square  of  60,  or  3600 
times  less  than  it  is  at  the  earth,)  the  body  would  be- 
gin to  fall  very  slowly,  moving  the  first  second  only 
the  twentieth  part  of  an  inch.  Were  a  body  to  fall 
toward  the  earth  from  the  greatest  possible  distance, 
the  velocity  it  would  acquire  would  never  exceed 
about  7  miles  in  a  second  ;  and  were  it  thrown  up- 
ward with  a  velocity  of  7  miles  per  second,  it  would 
never  return.  This,  however,  would  imply  a  velocity 
equal  to  about  twenty  times  the  greatest  speed  of  a 
cannon-ball. 

35.  When  a  body  is  thrown  directly  upward,  its  as- 
cent is  retarded  in  the  same  manner  as  its  descent  is 
accelerated  in  falling  ;  and  it  will  rise  to  the  height 

exemplified  1  Case  of  a  tree  leaping  from  a  trough.  How  is  the 
force  of  gravity  at  great  distances  from  the  earth  *?  How,  4000  miles 
off?  How,  at  the  distance  of  the  moon  1  State  the  law  by  which 
gravity  decreases.  What  velocity  would  a  body  acquire  by  falling 
from  the  greatest  possible  distance  1  How  far  would  it  go  it  thrown 
upward  with  a  velocity  of  7  miles  per  second  1 


MECHANICS. 


41 


Fig.  10. 


from  which  it  would  have  fallen  in  order  to  acquire  the 
velocity  with  which  it  is  thrown  upward. 

36.  VIBRATORY  MOTION.  Vibratory  motion  is  that 
which  is  alternately  backward  and  forward,  like  the 
motion  of  the  pendulum  of  a  clock. 
A  pendulum  performs  its'  vibrations 
in  equal  times,  whether  they  are  long^ 
or  short.  Thus,  if  we  suspend  two 
bullets  by  strings  of  exactly  equal 
lengths,  and  make  one  vibrate  over  a 
small  arc  and  the  other  over  a  large 
arc,  they  will  keep  pace  with  each 
other  nearly  as  well  as  when  their 
lengths  of  vibration  are  equal .  Long 
pendulums  vibrate  slower  than  short 
ones,  but  not  as  much  slower  as  the 
length  is  greater.  A  pendulum,  to 
vibrate  seconds,  must  be  four  times 
as  long  as  to  vibrate  half  seconds  ; 
to  vibrate  once  in  ten  seconds  it 
must  be  a  hundred  times  as  long  as 
to  vibrate  in  one  second,  the  com- 
parative slowness  being  proportional 
to  the  square  of  the  length.  The  motion  of  a  pendu- 
lum is  caused  by  gravity.  If  we  draw  a  pendulum  out 
of  its  position  when  at  rest,  and  then  let  it  fall,  it  will 
descend  again  to  the  lowest  point,  but  will  not  stop 
there,  for  the  velocity  which  it  acquires  in  falling  will 
be  sufficient,  on  account  of  its  inertia,  to  carry  it  to  the 
same  height  on  the  other  side,  (Art.  35,)  whence  it 
will  return  again  and  repeat  the  same  process ;  and 
thus,  were  it  not  for  the  resistance  of  the  air,  and  the 


35.  When  a  body  is  thrown  upward,  in  what  manner  is  it  re- 
tarded 1    How  high  will  it  rise  1 

36.  Define  vibratory  motion.     How  are  the  times  of  vibration  of 
a  pendulum  1    Example.    How  much  longer  is  a  pendulum  that 
vibrates  seconds,  than  one  that  vibrates  half  seconds  1    How  much 

4* 


42  NATURAL  PHILOSOPHY. 

friction  at  the  center  of  motion,  the  vibration  would 
continue  indefinitely. 

37.  It  is  the  equality  in  the  vibrations  of  a  pendu- 
lum, which  is  the  foundation  of  its  use  in  measuring 
time.     Time  may  be  measured  by  any  thing  which  di- 
vides duration  into  equal  portions,  as  the  pulsations  of 
the  wrist,  or  the  period  occupied  by  a  portion  of  sand 
in  running  from  one  vessel  to  another,  as  in  the  hour- 
glass ;  but  the  pendulum  can  be  made  of  such  a  length 
as  to  divide  duration  into  seconds,  an  exact  aliquot 
part  of  a  day,  and  is  therefore  peculiarly  useful  for  this 
purpose.     Since,  also,  the  pendulum  which  vibrates 
seconds  at  any  given  place,  is  always  of  the  same  in- 
variable length,  it  forms  the  best  standard  of  measures 
by  which  all  others  used  by  society  can  be  adjusted 
and  verified. 

38.  PROJECTILE   MOTION,     A   body   projected   into 
the  atmosphere,   rises  and  falls  in  a  curve  line,   as 
when  a  stone  is  thrown,  or  an  arrow  shot,  or  a  can- 
non ball  fired.     The  body  itself  is  called  a  projectile* 
the  curve  it  describes,  the  path  of  the  projectile,  and 
the   horizontal  distance  between  the  points  of  ascent 
and  descent,  the  range.     When  an  arrow  is  shot,  the 
impulse,  if  it  were  the  only  force  concerned,  would 
carry  it  forward  uniformly  in  a  straight  line ;  but  the 
gravity  continually  bends  its  course  toward  the  earth 
and  makes  it  describe  a  curve.     An  arrow,  (or  any 
missile,)  will  have  the  greatest  range  when  shot  at  an 
angle   of  45°  with  the  horizon  ;  and  the   range    will 
be  the  same   at   any  elevation  above   45°  as  at  the 
same    number   of    degrees    below    45°.      A    cannon 

longer  to  vibrate  in  10  seconds  than  in  1 1    What  causes  the  motion 
of  a  pendulum  1    Why  does  it  not  vibrate  forever  1 

37.  On  what  property  of  the  pendulum  is  its  use  for  measuring  time  1 
What  other  modes  are  there  1  Why  is  the  pendulum  better  than  other 
modes  1  On  what  principle  does  it  become  a  standard  of  measures'] 

38.  When  is  a  body  called  a  projectile  1    What  is  the  curve  de- 
scribed called  1    The  horizontal  distance  1   At  what  angle  of  eleva- 


MECHANICS. 


43 


ball   shot  at  an  elevation  of  60°    will   fall   at  the 

same  distance  from  the  gun  as  when  shot  at  an  angle 

of  30°.     Thus,  in  the  annexed  diagram,   a   ship   is 

Fig.  11> 


fired  on  from  a  fort,  as  she  is  attempting  to  pass  it* 
The  ball  fired  at  an  elevation  of  45°,  is  the  only  one 
that  reaches  the  ship  :  the  others  fall  short,  and  equally 
when  aimed  above  and  below  45°. 

39.  If  a  cannon  ball  were  fired  horizontally  from  the 
top  of  a  tower,  in  the  direction  of  P  B,  the  range  would 
depend  on  the  strength 
of  the   charge.     With  B        P 

an  ordinary  charge,  it 
would  descend  in  the 
curve  P  D ;  with  a 
stronger  charge,  it 
would  move  nearer  to 
the  horizontal  line  and 
descend  in  PE.  We 
may  conceive  of  the 
force  being  sufficient  to 
carry  the  ball  quite 
clear  of  the  earth,  and 
make  it  revolve  around 
it  in  the  circle. 

tion  must  an  arrow  be  shot,  to  have  the  greatest  range  1    At  what 
two  angles  would  the  ranges  be  equal  1 
39.  Explain  Figure  12. 


44 


NATURAL  PHILOSOPHY. 


SEC.  3.  —  Of  the  Center  of  Gravity. 

40.  The  center  of  gravity  of  a  body  is  a  certain 
point  about  which  all  parts  of  the  body  balance  each 
other,  so  that  when  that  point  is  supported,  the  whole 
body  is  supported.  If  across  a  perpendicular  support^ 


Fig.  13. 


' 


as  G,  (Fig.  13,)  I  lay  a  wire 
having  a  ball  at  each  end,  B  C, 
there  is  one  point  in  the  wire, 
and  only  one,  upon  which  the 
balls  will  balance  each  other.  This  point  is  the 
center  of  gravity  of  all  the  matter  contained  in  the 
wire  and  both  balls.  It  is  as  much  nearer  the  larger, 
B,  as  the  weight  of  this  exceeds  that  of  C.  When  two 
boys  balance  one  another  at  the  ends  of  a  rail,  the 
lighter  boy  will  require  his  part  of  the  rail  to  be  as 
much  longer  as  his  weight  is  less.  The  center  of 
gravity  of  a  regular  solid,  as  a  cube,  or  a  sphere,  lies 
in  the  center  of  the  body,  when  the  structure  of  the 
body  is  uniform  throughout  ;  but  when  one  side  is 
heavier  than  another,  the  center  of  gravity  lies  toward 
the  heavier  side. 

41.    The  line  of  direction  is  a  line  drawn  from  the 
center  of  gravity  of  a   body  perpendicularly   to   the 
Fig.  14.  Fig.  15. 


F  F 

horizon.     Thus,  G  F,  (Fig.  14  or  15,)  is  the  line  of 
direction.     When  the  line  of  direction  falls  within  the 

40.  Define  the  center  of  gravity.     Explain  Figure  13. 

41.  Explain  Figure  14.    Where  is  the  center  Of  gravity  of  a  regu- 
lar figure  fcjluciled  '* 


MECHANICS. 


45 


Fig.  16. 


base,  (as  in  Fig.  14,)  or  part  of  the  body  on  which  it 
rests,  the  body  will  stand ;  when  this  line  falls  without 
the  base,  (as  in  Fig.  15,)  the  body  will  fall.  At  Pisa, 
in  Italy,  is  a  cele- 
brated tower,  called 
the  leaning  tower.  It 
stands  firm,  although 
it  looks  as  though  it 
would  fall  every  mo- 
ment ;  and  being  ve- 
ry high,  a  view  from 
the  top  is  very  exci- 
ting. Yet  there  is  no 
danger  of  its  falling, 
because  the  line  of 
direction  is  far  with- 
in the  ba$e.  To  ef- 
fect this,  the  lower 
part  of  the  tower  is 
made  broader  than 
the  upper  parts,  and  ^^gjj^ 
of  heavier  materials. 
These  two  precautions  carry  the  center  of  gravity  low. 
Structures  in  the  form  of  a  pyramid,  as  the  Egyptian 
pyramids,  have  great  firmness,  because  the  line  of  di- 
rection passes  so  far  within  the  base. 

42.  If  we  stick  a  couple  of  pen- 
knives in  a  small  bit  of  wood,  and  poise 
them  on  the  finger,  or  adjust  them 
so  that  the  center  of  gravity  will  fall 
in  the  line  of  a  perpendicular  pin,  the 
point  of  the  wood  will  rest  firmly  on 
the  head  of  the  pin,  so  that  the  knives 
may  be  made  to  vibrate  on  it  up  and 
down,  or  to  revolve  around  it,  with- 


Fig.  17. 


Define  the  line  of  direction.    Explain  Fig.  16,  Tower  of  Pisa. 
Why  are  pyramids  so  firm  1 


46 


NATURAL  PHILOSOPHY. 


Fig.  18. 


out  falling  off.  A  loaded  ship  is  not  easily  over- 
turned, because  the  center  of  gravity  is  so  low,  that 
the  line  of  direction  can  hardly  be  made  to  fall  with- 
out the  base  ;  but  a  cart  loaded  with  hay  or  bales  of 
cotton  is,  on  the  other  hand,  easily  upset,  because 
the  center  of  gravity  is  so  high.  A  stage  coach 
carrying  passengers  or  baggage  on  the  top,  is  much 
more  liable  to  upset  than  it  is  when  the  load  is  all  on 
a  level  with  the  wheels.  A  round  ball,  however 
large,  will  rest  firmly  on 
a  very  narrow  base,  be- 
cause the  center  of  grav- 
ity (which  is  in  the  cen- 
ter of  the  ball)  is  always 
directly  over  the  point  of 
support ;  and,  according 
to  the  definition,  when ' 
this  is  supported,  the  bo- 
dy is  supported.  In  the 
annexed  diagram,  a  hea- 
vy ball,  connected  with 
the  figure,  bends  under 
the  table,  and  thus  brings 
the  center  of  gravity  of 
the  whole  within  the  base, 
so  that  the  animal  rests  firmly  on  his  hind  legs. 

43.  Animals  with  four  legs  walk  sooner  and  more 
firmly  than  those  with  only  two,  because  the  line  of 
direction  is  so  much  more  easily  kept  within  the 
base.  Hence,  children  creep  before  they  walk,  and 
the  art  of  walking,  and  even  of  standing  firmly,  re- 
quires so  nice  an  adjustment  of  the  center  of  gravity, 
(which  must  always  be  kept  over  the  narrow  base 

42.  Explain  Fig.  18.    Why  is  not  a  loaded  ship  easily  overturn- 
ed *?    A  cart  loaded  with  hay — a  stage  coach— a  round  ball  1 

43.  Why  do  four-legged  animals  walk  sooner  than  two-legged  7 
Why  do  children  creep  before  they  walk  1 


MECHANICS.  47 

within  the  feet,)  that  it  is  learned  only  after  much  ex- 
perience.  Children  at  school,  also,  are  sometimes  di- 
rected to  turn  out  their  toes  when  they  walk,  and  to 
extend  one  foot  from  the  other  in  taking  a  position  to 
speak,  because  such  attitudes,  allowing  a  broader 
base  for  the  line  of  direction,  appear  more  firm  and 
dignified. 

44.  A  boy  promised  another  a  cent,  if  he   would 
pick  it  up  from  the  floor,  standing  with  his  heels  close 
against  the  wall.     But  in  attempting  to  pick  it  up, 
he  pitched  upon  his  face.     Performances  on  the  slack 
rope,  which  often  exhibit  astonishing  dexterity,  depend 
upon  a  skilful   adjustment  of  the  center   of  gravity. 
The  process  is  sometimes   aided    by    holding   in   the 
hand  a  short  stick  loaded  with  lead,  which  is  so  flour- 
ished on  one  side  or  the  other,  as  always  to  keep  the 
center  of  gravity  over  the  narrow  base.     Among  the 
ancients,  elephants  were  sometimes  trained  to  walk  a 
tight  rope ;  a  feat  which  was  extremely  difficult  on 
account  of  the  great  weight  of  the  animal. 

45.  Bodies  subject  to   no   other   forces   than   their 
mutual  attraction,  and  in  a  situation  'to  approach  each 
other  freely,  will  meet   in   their   common  center  of 
gravity.     If  the  earth  and  moon  were  left  to  obey  fully 
their  attraction  for  each  other,  they  would  immediate- 
ly begin  to  approach  each  other  in  a  direct  line,  mov- 
ing slowly  at  first,  but  swifter  and  swifter,  until  they 
would  meet  in  their  common  center  of  gravity,  which 
would  have  its  situation  as  much  nearer  to  the  earth 
as  the   weight  of  the    earth   is  greater  than   that  of 
the  moon.     So  all  the  planets  and  the  sun,  i£  aban- 
doned to  their- mutual  attraction,  would  rush  together 
to   a  common    point,    which   on   account   of  the  vast 
quantity  of  matter  in  the  sun,  lies  within  that  body. 

44.  Case  of  picking  up  the  cent.    Performances  on  the  slack  rope 
by  men,  and  even  by  elephants,  explained. 

45.  Where  will  bodies  meet  by  their  mutual  attraction  1  Examples 


48  NATURAL  PHILOSOPHY. 

Indeed,  were  all  the  bodies  in  the  universe  abandoned 
to  their  mutual  attraction,  they  would  meet  in  their 
common  center  of  gravity. 

SECTION  4. — Of  the  Principles  of  Machinery. 

46.  The  elements  of  all  machines  are  found  among 
the  Mechanical  Powers,  which  are  six  in  number — the 
Lever,  the  Wheel  and  Axle,  the  Pulley,  the  Screw, 
the   Inclined   Plane,    and   the  Wedge.     That   which 
gives  motion  is  called  the  power  ;  that  which  receives 
it,  the  weight.     The  first  inquiry  is,  what  power,  in 
the  given  case,  is  required  just  to  balance  the  weight. 
Any  increase  of  power  beyond  this,  would  of  course 
put  the  weight  in  motion.     It  is  a  general  principle  in 
Machines,  that  the  power  balances  the  weight  when  it 
has  just  as  much  momentum.     Now   we  may   give   a 
small  power  as  much  momentum  as  a  great  weight,  by 
making  it  move  over  as  much  greater  space  in  the 
same  time,   as  its  quantity   of  matter   is   less.     One 
ounce  may  balance  a  thousand  ounces,  if  the  two  be 
connected  together  in   such  a  way  that  the  smaller 
mass,  when  they  are  put  in  motion,  moves  a  thousand 
times  as  fast  as  the  larger.     If  the  momentum  of  the 
power  be  increased  beyond  that  of  the  weight,  as  may 
be  done  by  increasing  its  quantity  of  matter,  then  it 
will  overcome  the  weight  and  make  it  move  with  any 
required  velocity.     Whatever  structure  connects  the 
power  and  the  weight  is  a  machine. 

47.  THE  LEVER.    Figure  19  represents  a  lever  of  the 
simplest  kind,  where  P  is  the  power,  W  the  weight,  and 

in  the  moon  and  earth,  and  all  the  bodies  of  the  solar  system — final- 
ly, all  the  bodies  in  the  universe. 

46.  What  are  the  elements  of  all  Machines  1  Enumerate  the  six  Me- 
chanical powers.  Distinguish  between  the  power  and  the  weight  ? 
What  is  the  first  inquiry  respecting  the  power  1  What  is  a  general 
principle  in  Machines,  respecting  momentum  ?  How  may  we  give 
a  small  power  as  much  momentum  as  a  great  weight  1  How  may 
one  ounce  balance  a  thousand  1  What  happens  when  the  momen- 
tum of  the  power  is  increased  beyond  that  of  the  weight  1  What 
does  any  structure  that  connects  the  power  and  the  weight  become  1 


MECHANICS.  49 

F  the  fulcrum,  or  point  of  support.  Now  P  will  just 
balance  W  when  its  weight 

is  as  much  less  as  its  dis-      lg<     '      

tance  from  the  fulcrum  ispg  |r 

greater.  For  example,  if  it 
is  three  times  as  far  from 
the  fulcrum  as  W,  then  one 

pound  will  balance  three ;  three  pounds  will  balance 
nine  ;  and,  universally,  in  an  equilibrium,  the  power 
multiplied  into  its  distance  from  the  fulcrum,  mil  equal 
the  weight  multiplied  into  its  distance.  In  the  present 
case,  where  the  longer  arm  of  the  lever  is  three  times 
the  length  of  the  shorter,  a  power  of  ten  pounds  will 
balance  a  weight  of  thirty. 

Fig.  20. 


48.  This  principle  is  exemplified  in  a  common  pair 
of  steel-yards.  The  same  power  is  made  to  bal- 
ance different  weights  of  merchandise  by  attaching 
W  to  the  shorter  and  P  to  the  longer  arm,  and  placing 
P  in  a  notch  that  is  as  much  farther  from  the  fulcrum 


47.  Explain  Fig.  19.    State  the  general  principle  of  the  equilibri- 
um o.f  the  lever.    Examples. 

48.  Explain  the  principle   of  Steel-yards.      How   is  the   same 
power  made  to  balance  different  weights  1    Explain  the  difference 

5 


50  NATURAL    PHILOSOPHY. 

as  its  weight  is  less  than  that  of  the  merchandise, 
W.  Steel-yards  have  commonly  a  smaller  and  a 
larger  side ;  the  former  being  ounce,  and  the  latter 
quarter-pound  notches.  On  examining  such  a  pair  of 
steel-yards,  it  will  be  seen  that  the  hook  to  which  the 
merchandise  is  attached,  is  four  times  as  far  from  the 
fulcrum,  when  we  weigh  on  the  small,  as  when  we 
weigh  on  the  large  side.  Hence,  we  have  to  move 
the  counterpoise  over  four  notches  on  this  side  to  gain 
as  much  povyer  as  we  gain  in  one  notch  on  the  other. 
The  spaces  over  which  the  power  and  weight  move 
respectively,  are  in  the  same  proportion.  Thus,  when 
the  counterpoise  is  made  to  balance  a  weight  ten 
times  as  large  as  itself,  it  will  be  seen,  by  making  the 
arm  of  the  steel-yards  vibrate  up  and  down,  that  the 
counterpoise  moves  ten  times  farther,  in  the  same  time, 
than  the  weight  does,  and  of  course  with  ten  times  the 
velocity.  Hence  the  momenta  of  the  power  and  the 
weight  are  the  same.  A  crow-bar  illustrates  the  same 
principle,  when  a  man  lifts  a  weight  much  heavier 
than  the  amount  of  force  he  applies,  by  making  that 
force  act  at  the  longer  end  of  the  lever.  A  pair  of 
shears  is  formed  of  two  such  levers  combined;  and 
the  nearer  we  bring  the  article  to  be  cut  to  the  ful- 
crum, the  greater  is  the  mechanical  advantage  gained. 
Two  boys  differing  in  size,  moving  each  other  at  the 
end  of  a  pole  laid  across  the  fence,  exemplify  the  same 
principle. 

49.  In  the  foregoing  cases  the  weight  and  the  pow- 
er are  on  opposite  sides  of  the  fulcrum,  and  it  is  called 
a  lever  of  the  first  kind.  When  the  power  and 
weight  are  on  the  same  side  of  the  fulcrum,  but  the 
weight  nearer  to  it  than  the  povver,  it  is  a  lever  of  the 

between  the  smaller  and  the  larger  side.     Show  that  the  momenta 
of  the  counterpoise  and  weight  are  equal.     Examples  in  a  crow- 
bar— a  pair  of  shears — boys  on  a  rail. 
49.  Distinguish  between  levers  ofthe  first,  second,  and  third  kinds. 


MECHANICS.  51 

second  kind,  as  in  the  fol-      x Fig.  21. 

lowing  figure.  The  m 
chariical  advantage  gain 
here  is  the  same  as  in  the  L 
first,  for  the  power  moves 
as  much  faster  than  the 
weight  as  it  is  more  dis- 
tant from  the  fulcrum. —  -w 
When  the  power  and  weight  are  both  on  the  same 
side  of  the  fulcrum,  but  the  power  nearer  to  it  than 
the  weight,  it  constitutes  a  lever  of  the  third  kind,  as 

in  figure  22.     A  door  mo- 
> .     '  i  •  •  r  IP   2 

ving   on    its   hinges   is   a 

weight,     the     matter     ofF 
which,     for    our    present  2, 
purpose,  may  be  consider- 
ed as  all  collected  in  the  ^ 
center  of  gravity,  which, 

on  account  of  the  regular  figure  of  the  door,  is  the  cen- 
ter of  the  door  ;  and  the  effects  of  any  force  applied  to 
a  body  are  the  same  as  though  all  the  matter  was  con- 
centrated in  the  center  of  gravity,  and  the  force  was  ap- 
plied to  that  point.  Now  if,  in  shutting  the  door,  I  place 
my  hand  on  the  edge,  this  point  being  farther  from  the 
fulcrum  than  the  center  of  gravity,  I  gain  a  mechan- 
ical advantage,  because  the  power  moves  faster  than 
the  weight;  but  if  I  apply  my  hand  nearer  the  ful- 
crum than  the  center  of  gravity,  then  the  power  moves 
slower  than  the  weight,  and  operates  under  a  mechan- 
ical disadvantage ;  and  as  I  approach  nearer  and 
nearer  to  the  hinges,  the  door  is  shut  with  greater  and 
greater  difficulty.  In  the  former  case,  the  door  exem- 
plifies the  principle  of  a  lever  of  the  second  kind  ;  in 
the  latter,  of  the  third.  Suppose  a  ladder  to  lie  on  the 
ground,  and  it  is  required  to  raise  it  on  one  end  by 

How  may  a  door,  in  shutting,  be  either  of  the  second  or  third  kind! 
Example  in  a  ladder. 


52 


NATURAL  PHILOSOPHY. 


taking  hold  of  one  of  the  rounds.  If  I  take  hold  of 
the  lowest  round,  it  will  require  a  great  effort  to  raise 
it,  especially  if  the  ladder  is  long.  This  effort  will 
be  less  and  less,  until  I  come  to  the  middle  round, 
where  I  should  neither  gain  nor  lose  any  mechanical 
advantage,  but  should  lift  the  ladder  like  any  other 
body  of  the  same  weight,  if  raised  directly  from  the 
ground  by  a  string.  If  I  apply  my  hand  to  any  round 
beyond  the  middle,  toward  the  farther  end,  I  gain  a 
mechanical  advantage,  and  the  greater  as  I  approach 
nearer  to  the  end  of  the  ladder.  We  shall  leave  it  to 
the  ingenuity  of  the  pupil  to  account  for  these  several 
cases. 

Fig.  23. 


t  50.  THE  WHEEL  AND  AXLE. — The  figure  repre- 
sents a  wheel,  A  N  O,  and  axis,  L  M,  where  a  small 
power  w,  balances  a  greater  weight,  W.  The  power 
required  to  balance  the  weight  is  as  much  less  than 
the  weight  as  the  diameter  of  the  axle  is  less  than  that 
of  the  wheel.  The  wheel  and  axle  has  a  great  analo- 
gy to  the  lever,  and  is  indeed  little  more  than  a  re- 
volving lever.  For  if  the  power  were  applied  to  the 

50.  Explain  Figure  23.  How  much  less  is  the  power  than  the 
weight  1  Show  the  analogy  between  the  wheel  and  the  lever. 
Explain  Figure  24. 


MECHANICS.  53 

end  of  one  of  the  spokes  of  the  wheel,  that  spoke,  as  it 
revolved,  would  describe  the  figure  of  a  wheel.    Thus, 

Fig.  24. 


the  capstan  of  a  ship  is  a  large  upright  axle,  having 
holes  near  the  top  into  which  long  levers  are  inserted. 
The  men  press  upon  the  ends  of  these  and  gain  a  me- 
chanical advantage  in  proportion  as  the  length  of  the 
lever  exceeds  the  radius  of  the  axle.  By  this  means 
they  draw  up  heavy  anchors. 

51.  Wheels  are  much  employed  in  machinery,  and 
.serve  very  various  purposes,  although  they  do  not  al- 
ways act  upon  the  principle  of  the  wheel  and  axle,  as 
just  explained.  In  carriages,  their  chief  use  is  to 
overcome  friction,  since  a  body  that  rolls  on  the 
ground  meets  with  much  less  resistance  than  one  that 
slides  ;  and  in  lifting  a  wheel  over  an  obstacle,  as  a 
stone,  a  mechanical  advantage  is  gained  in  the  same 
proportion  as  the  radius  of  the  wheel  exceeds  that  of 
the  axle.  Large  wheels,  therefore,  overcome  obstacles 
better  than  small  ones.  Wheels  are  much  employed 
also  to  regulate  velocity.  Just  step  into  a  mechanic's 
shop  and  see  this  use  exemplified  in  the  turner's  lathe. 
By  passing  a  band  over  a  large  wheel  that  turns  with  a 
steady  motion,  one  may  convey  that  motion  to  the  small 

51.  What  is  the  use  of  wheels  in  carriages'?    What  advantage  is 
gained  by  rolling  instead  of  sliding  '"?  Also,  in  overcoming  obstacles  1 
Which  gain  most,  large  or  small  wheels  1  Use  of  wheels  in  regula- 
rs* 


54 


NATURAL  PHILOSOPHY. 


wheel  of  a  lathe,  and  the  smaller  wheel  will  revolve 
as  much  faster  than  the  larger  as  its  diameter  is  less. 
Now  by  using  small  wheels  of  different  diameters  on 
the  lathe,  we  may  increase  or  diminish  the  velocity  at 
pleasure.  The  same  principle  is  illustrated  in  a  com- 
mon  spinning  wheel,  and  in  machinery  for  spinning 
cotton. 

52.  In  clock-work,  there  is  usually  a  combination  of 
a  number  of  wheels,  where  one  wheel  is  connected 
to  the  axis  of  another  by  a  small  wheel  fastened  to  the 
called  a  pinion.  Thus,  the  three  wheels,  A,  B, 


axs, 


Fig.  25. 


C,  are  connected.  The  power  is  applied  to  the  whee 
A,  on  whose  axis  is  the  pinion  a,  the  teeth  of  whicl 
or  leaves,  as  they  are  called)  catch  into  the  teeth  of 
~,  whose  pinion  b  in  like  manner  turns  the  wheel  C. 
Here  the  motion  of  each  succeeding  wheel  is  less 
than  the  preceding  ;  for  if  the  pinion  a  have  ten  leaves, 
and  the  wheel  B  100  teeth,  the  pinion  in  turning  once 
would  catch  but  ten  teeth  of  the  wheel,  and  must  there^ 
fore  turn  ten  times  to  turn  B  once.  If  the  pinion  £  has 

ting  velocity.    How  exemplified  in  a  turner's  lathe  1    In  a  common 
spinning  wheel  1 
52.  Explain  the  use  of  wheels  in  clock-work.    Explain  Fig.  25. 


MECHANICS. 


55 


also  10  leaves,  and  the  wheel  C  100  teeth,  then  C 
turns  ten  times  as  slow  as  B  and  a  hundred  times  as 
slow  as  A.  By  altering  the  proportions  between  the 
number  of  teeth  in  the  wheel  and  leaves  in  the  pinion, 
we  may  alter  the  velocity  of  a  wheel  at  pleasure ; 
and  this  is  the  way  in  which  wheels  are  made  to  move 
faster  or  slower,  at  any  required  rate,  in  clocks  and 
watches.  If  we  apply  the  power  at  the  other  end  and 
let  the  wheel  C  act  on  the  pinion  b,  and  the  wheel  B 
on  the  pinion  a,  then  B  will  turn  ten  times  as  fast  as 
C,  and  A  ten  times  as  fast  as  B,  and  a  hundred  times 
as  fast  as  C  ;  so  that,  when  the  wheels  carry  the  pin- 
ions,  the  velocity  is  increased,  but  when  the  pinions 
.carry  the  wheels,  it  is  diminished. 

53.  THE  PULLEY. — A  pulley  is  a  grooved  wheel, 
around  which  a  rope  is  passed,  and  is  either  fixed  or 
^novable.  Figure  26  represents  a  fixed  pulley  ;  and 
Fig.  26.  Fig.  27. 


W 


Show  how  the  motion  is  accelerated  in  one  direction  and  retarded  in 

the  other.    How  may  we  alter  the  velocity  of  a  wheel  at  pleasure  1 

53.  Define  the  Pulley.    Name  the  two  kinds.    What  is  the  use  of 


NATURAL  PHILOSOPHY. 


Fig.  28. 


here  no  mechanical  advantage  is  gained,  since  the 
power  moves  just  as  fast  as  the  weight,  and  we  must 
remember  that  it  is  only  when  the  power  moves  faster 
than  the  weight,  that  any  mechanical  advantage  is 
gained.  The  boy,  however,  in  figure  27,  draws 
himself  up  by  lifting  only  half  his  weight,  because  the 
two  ropes  support  equal  portions  of  the  weight.  The 
principal  use  of  the  fixed  pulley  is  to  change  the  di- 
rection of  the  weight.  Thus,  in  drawing  a  bucket  out 
of  a  well,  it  is  more  convenient  to  pull  downward  by  a 
rope  passing  over  a  pulley  above  the  head,  than  upward 
by  drawing  directly  at  the  bucket.  By  the  movable 
pulley  we  gain  a  mechanical  advantage,  for  by  this  we 
can  give  the  weight  a  slower 
motion  than  the  power  has,  and 
can  proportionally  increase  the 
efficacy  of  the  power.  Thus, 
in  figure  28,  as  both  the  ropes. 
A  arid  E,  are  shortened  as  the 
weight  ascends,  the  rope  to 
which  P  is  attached  is  length- 
ened by  both,  and  therefore  P 
descends  twice  as  fast  as  W 
rises,  and  the  efficacy  of  the 
power  is  doubled.  By  employ- 
ing a  pulley  with  a  number  of 
grooves  (called  a  block)  with 
a  rope  around  each,  we  may 
make  the  power  run  off  a  great 
length  of  rope  while  the  weight 
rises  but  little,  being  equal  to 
the  combined  length  by  which  all  the  ropes  of  the  block 
are  shortened.  Thus,  if  the  block  carries  twelve  ropes, 
the  power  is  increased  in  efficacy  12  times.  Instead 
of  a  single  block  with  a  number  of  grooves,  several 


\V 


the  fixed  pulley  1    Of  the  movable  pulley  1    Explain  the  power  of 
a  block  of  pulleys. 


MECHANICS.  57 

pulleys  with  single  grooves  are  combined  upon  a  simi- 
lar principle.  By  a  block  of  pulleys,  two  men  will 
lift  a  rock  out  of  a  quarry  a  thousand  times  as  heavy 
as  they  could  lift  with  their  naked  hands ;  but  the  rope 
at  which  they  pull  will  run  off  a  thousand  times  as  fast 
as  the  weight  rises. 


54.  THE  INCLINED  PLANE. — The  Inclined  Plane  be- 
comes a  mechanical  power  in  consequence  of  its  sup- 
porting a  part  of  the  weight,  and  of  course  leaving  only 
a  part  to  be  supported  by  the  power.  If  a  plank,  for  ex- 
ample, having  on  it  a  cannon  ball,  is  laid  flat  on  the 
ground,  it  supports  the  whole  weight  of  the  ball.  If 
one  end  is  gradually  raised,  more  and  more  force  must 
be  applied  to  keep  the  ball  from  rolling  down  the 
plane :  and  when  the  plank  becomes  perpendicular,  a 
force  would  be  required  to  sustain  the  ball  equal  to  its 
whole  weight.  We  may  therefore  diminish  the  ef- 
fect of  gravity,  in  ascending  from  one  level  to  another, 
as  much  as  we  please,  by  making  the  inclination  of  the 
plane  small.  A  builder  who  was  erecting  a  large  ed- 
ifice, had  occasion  at  last  to  raise  heavy  masses  of  stone 
to  the  height  of  sixty  feet.  He  might  have  hauled 
them  up  by  pulleys  ;  but  this  was  inconvenient,  and  be- 

54.  How  does  the  Inclined  Plane  become  a  mechanical  power  ?  Ex- 


58 


NATURAL  PHILOSOPHY. 


sides,  pulleys  are  subject  to  so  much  friction  as  to  oc- 
casion a  great  loss  of  power.  He  therefore  con- 
structed of  timbers  and  planks,  an  inclined  plane  six 
hundred  feet  long,  and  conveyed  the  blocks  of  stone 
up  them  on  rollers.  As  the  plane  was  ten  times  as 
long  as  it  was  high,  it  was  as  easy  to  roll  1000  pounds 
up  the  plane  as  it  would  have  been  to  draw  up  100 
pounds  by  a  fixed  pulley.  But  as  the  plane  was  ten 
times  as  long  as  it  was  high,  the  weight  would  have  to 
pass  over  ten  time's  the  space  that  it  would  if  it  had 
been  raised  perpendicularly  by  the  pulley.  In  all 
cases,  the  mechanical  advantage  gained  by  the  inclined 
plane  is  in  the  same  proportion  as  its  length  exceeds 
its  height.  When  a  horse  draws  a  loaded  cart  on  lev- 
el ground,  he  has  merely  the  friction  to  overcome  ;  but 
when  he  drags  it  up  hill,  he  has,  besides  the  friction,  to 
lift  a  certain  part  of  the  load,  which  part  will  be  great- 
er in  proportion  as  the  hill  is  steeper.  If  the  rise  is 
one  part  in  ten,  then  he  would  lift  one  tenth  of  the  load 
continually. 

55.  The  SCREW. — The  screw  is  represented  in  the 
following  diagram  as  acting 
upon  a  press,  which  is  a  very 
common  use  that  is  made  of  it. 
As  the  screw  is  turned,  it  ad- 
vances lengthwise  through  a 
space  just  equal  to  the  dis- 
tance between  the  threads. 
Now  if  the  power  be  applied 
directly  to  the  head  of  the 
screw,  then,  in  turning  the 
screw  once  round,  the  power 
would  move  over  as  much 
more  space  than  the  screw  advances,  as  the  circum- 

ample.     How  employed  in  building  1    What  makes  it  so  hard  to 
draw  a  load  up  hill  q. 
55.  Explain  Fig.  30.    How  is  the  mechanical  advantage  gained, 


Fig.  30. 


MECHANICS.  59 

Terence  of  the  head  is  greater  than  the  distance  be- 
tween the  threads.  The  mechanical  advantage  gained 
is  in  the  same  proportion ;  and  we  may  increase  the 
efficacy  of  the  power  either  by  lessening  the  distance 
between  the  threads,  or  by  increasing  the  space  over 
which  the  power  moves.  If  we  attach  a  lever  to  the 
head  of  the  screw,  and  apply  the  hand  at  the  end, 
then  we  make  the  power  move  over  a  space  vastly 
greater  than  that  through  which  the  screw  advances, 
and  the  force  becomes  very  powerful,  and  will  urge 
down  the  press  upon  the  books,  or  any  thing  in  press, 
with  great  energy. 

56.  THE  WEDGE. — The  Wedge  is    an   instrument 
used    for   separating   bodies,    or  the  parts  of   bodies, 
from  each   other,   as  is  seen  in  the    common  wedge 
used  for  splitting  rocks  or  logs  of  wood.     In  the  kind 
of  wedge  in  ordinary  use,  the  mechanical  advantage 
gained  is  greater  in  proportion  as  the  wedge  is  thinner. 
Accordingly,  it  requires  but  a  small  force  to  drive  a  thin 
wedge,  but  a  greater  force  in  proportion  as  the  thick- 
ness increases.     Cutlery  instruments,  as  knives,  axes, 
and  the  like,  act  on  the  principle  of  the  wedge.     When 
long   and    proportionally  thin,  the  wedge    becomes  a 
mechanical   power  of  great  force,  sufficient  to  raise 
ships  from  their  beds. 

57.  MACHINES. — Machines  are  compounded  of  the 
mechanical  powers  variously  united.     We  recognise, 
at  one  time,  the  union  of  the  lever  with  the  screw  ;  at 
another,  of  the  wheel  and  axle  with  the  pulley ;  and, 
at  another,  of  nearly  all  the  mechanical   powers  to- 
gether.    The  following  figure  represents  a   machine 
for  hauling   a  vessel   on   the    stocks,   combining  the 
wheel  and  axle,  the  screw,  the  inclined  plane,  and  the 
pulley.     Each  contributes  to  increase  the  efficacy  of  the 

when  the  power  is  applied  to  the  head  of  the  screw!    Also  when 
applied  at  the  end  of  the  lever  ? 

56.  What  is  the  Wedge  used  for  1  How  is  the  mechanical  advan- 
tage of  i he  \vedge  incrca  >d  .' 


60 


NATURAL   PHILOSOPHY. 


force,  and  all  together  make  a  powerful  machine.     A 

man  applies  his  hand  at  B,  and  turns  a  crank  which 

Fig.  31. 


acts  on  the  principle  of  the  lever  upon  the  screw  at  D- 
If  the  space  over  which  the  hand  moves  in  one  revo- 
lution is  a  hundred  times  as  great  as  the  distance  be- 
tween the  threads  of  the  screw,  then  the  mechanical 
advantage  gained  is  in  the  same  proportion,  and  the 
force  with  which  the  screw  urges  the  teeth  of  the 
wheel,  is  a  hundred  times  that  applied  by  the  hand  to 
the  crank.  The  diameter  of  the  wheel  is  four  times 
that  of  the  axle ;  therefore,  the  force  applied  at  E  is 
four  hundred  times  that  at  B.  This  acts  on  a  com- 
bination of  pulleys,  which,  having  four  ropes,  multi- 
ply it  again  four  times,  and  it  becomes  sixteen  hun- 
dred. The  inclined  plane  is  twice  as  long  as  it  is 
high,  and  therefore  doubles  the  efficacy  of  the  power, 
and  it  becomes  three  thousand  and  two  hundred  times 
what  it  was  originally.  So  that  the  single  force  which 
a  man  can  exert  by  means^of  such  a  machine  is  pro- 
digious ;  and  if  the  machine  was  so  contrived  (as  it 
might  easily  be)  that  a  pair  of  horses  or  a  yoke  of 

57.  How  are  Machines  composed  1   PIxplain  Fi£.  31.   How  would 
the  velocity  of  the  weight  compare  with  that  of  the  power'? 


MECHANICS.  61 

cattle,  instead  of  the  man,  could  turn  the  machine,  the 
force  would  be  adequate  to  move  the  largest  ship. 
Such  a  machine,  however,  would  move  the  body  with 
extreme  slowness.  Its  motion,  in  fact,  would  be  dimin- 
ished as  much  as  the  efficacy  of  the  power  was  in- 
creased. This,  as  we  have  said  before,  is  a  universal 
principle  in  mechanics  ;  so  that  we  may  find  the  power 
exerted  by  any  machine,  by  seeing  how  much  faster 
the  moving  force  goes  than  the  weight. 

58.  Machines,  therefore,  gain  no  momentum:  the 
power  multiplied  into  its  velocity  always  equals  the 
weight   multiplied    into    its    velocity.     But   although 
machines  do  not  of  themselves  generate  any  force,  they 
enable  us  to  apply  it  to  much  greater  advantage — to 
change  its  direction  at  pleasure — to  regulate  its  ve- 
locity— and  to  bring  in  to  the  aid  of  the  feeble  powers 
of  man  the  energies  of  the  horse  and  the  ox,  of  water, 
wind,  and  steam. 

59.  FRICTION. — The  principles  of  machinery  are  first 
investigated,  on  the  supposition   that  machines  move 
without   resistance   from  external  causes.     Then  the 
separate  influence  of  such  accidental  causes  of  irreg- 
ularity, in  any    given    case,  is  ascertained   and   ap- 
plied.    The  two  most   general    impediments   to   ma- 
chines are  friction  and  resistance  of  the  air,  which 
occasion  more  or  less  destruction  of  force  in  all  ma- 
chines.    Friction   arises    from   the    resistance   which 
different  surfaces  meet  with  in  moving  on  each  other. 
Perfectly  smooth  surfaces  adhere  together  by  a  cer- 
tain force,  opposing  a  corresponding  resistance  to  the 
motion  of  the  surfaces x on  one   another;  but  the  as- 
perities  which   exist    on    most    surfaces    occasion   a 
much  greater  resistance.     An  extreme  case  is  when 


58.  Do  Machines  gain  any  momentum  1    "What  two  products  are 
always  equal  to  each  other  1    How  do  machines  aid  usl 

59.  On  what  supposition  are  the  principles  of  machinery  first  inves- 
tigated 1    What  are  the  two  rno.st  general  impediments  to  machines  1 

(5 


62 


NATURAL    PHILOSOPHY. 


Fig.  32. 


one  brush  is  slid  across  another,  and  the  hairs  inter- 
lace. By  careful  experiments  on  friction,  the  follow- 
ing are  found  to  be  its  principal  laws.  First,  the 
friction  of  a  body,  other  things 
being  equal,  is  proportioned  to 
its  weight.  If  a  brick  is  laid 
on  a  table,  with  a  string  attach- 
ed to  it  connected  with  a  scale 
below,  by  placing  weights  in 
the  scale  we  may  ascertain 
just  how  much  force  it  takes 
to  drag  it  off  from  the  table 
under  different  circumstances,  and  this  will  be  the 
measure  of  the  friction.  We  should  suppose  that  the 
friction  would  be  greater  on  its  broad  than  on  its 
narrow  side ;  but  experiments  show  that  it  is  equal  in 
the  two  cases,  so  that  extent  of  surface  makes  no  dif- 
ference when  the  weight  remains  the  same.  We 
may  let  the  same  brick  rest  on  either  side,  and  load 
it  with  different  weights,  equal  to  its  own  weight, 
double,  triple,  and  so  on.  In  all  cases,  we  shall  find 
the  friction  increased  in  the  same  proportion  as  the 
weight.  Secondly,  friction  is  increased  by  bodies 
remaining  some  time  in  contact  with  each  other ;  and 
when  the  contact  is  but  momentary,  as  when  a  body 
is  in  very  swift  motion,  the  amount  of  friction  is 
greatly  diminished.  Thus,  when  a  carriage  is  in 
swift  motion  over  a  road,  it  encounters  less  resistance 
from  friction  in  passing  a  given  distance,  than  when 
it  moves  slowly.  The  same  is  strikingly  the  case  in 
railway  cars. 

60.  Rolling  are  subject  to  far  less  friction  than 
sliding  bodies.  Thus,  if  a  coach  wheel  be  locked, 
that  is,  made  to  slide  down  hill  instead  of  rolling,  its 

What  causes  friction  7    State  an  extreme  case.    To  what  is  the  fric- 
tion of  a  body  proportioned  ?    How  is  the  amount  of  friction  affected 
by  continued  contact? 
60.  Difference  between  rolling  and  sliding  bodies  1   Use  of  lubrica- 


MECHANICS.  63 

friction  may  be  so  much  increased  as  to  check  the 
rapidity  of  "descent  in  any  required  degree.  Rollers 
are  therefore  employed  in  transporting  heavy  bodies, 
to  diminish  friction  ;  and,  for  the  same  purpose,  sur- 
faces are  made  smooth  by  applying  grease,  or  different 
pastes,  or  even  water,  all  of  which  fill  up  the  inequali- 
ties and  thus  diminish  the  asperities  of  the  surface. 
Although  friction  presents  a  resistance  to  machines, 
yet  it  has  its  uses  in  mechanical  operations.  It  is 
this  which  makes  the  screw  and  the  wedge  keep  their 
places  ;  and  it  is  the  friction  of  the  surfaces  of  brick 
and  stone  against  each  other,  which  gives  stability  to 
buildings  constructed  of  them.  The  wheels  of  a  car- 
riage advance  by  their  friction  against  the  ground. 
On  perfectly  smooth  ice  they  would  turn  without  ad- 
vancing. We  could  not  walk  did  not  friction  furnish 
us  with  a  foothold  ;  and,  it  is  for  want  of  friction  that 
walking  is  so  difficult  on  smooth  ice.  So  rail  cars 
meet  with  great  difficulty  in  proceeding  when  the  rails 
have  been  recently  rendered  slippery  by  ice :  the 
wheels  turn  without  advancing.  Friction  is  even  em- 
ployed as  a  mechanical  force,  as  when  a  lathe  is 
turned  by  the  friction  of  a  band.  Air  meets  with 
greater  resistance  in  passing  over  rough  surfaces  than 
water  does ;  for  water  deposites  a  film  of  its  own 
fluid  upon  the  surface  over  which  it  moves,  and  thus 
lubricates  it.  Hence  water  flows  in  pipes  with  less 
resistance  than  air  passes  over  the  surfaces  of  a  rough 
and  sooty  chimney. 

61.  The  resistance  which  bodies  meet  with  in 
passing  through  air  or  water,  increases  rapidly  as  the 
velocity  is  increased,  being  proportioned  to  the  square 
of  the  velocity.  Thus,  if  a  steamboat  doubles  its 

ting  substances  1  Give  examples  of  the  uses  of  friction  in  the  screw 
and  the  wedge — in  the  materials  of  a  building — in  carriage  wheels 
— in  lathes.  Which  meets  with  the  greater  resistance  from  friction, 
water  or  air  ? 
61.  How  is  friction  proportioned  to  velocity  ?  Example. 


64  NATURAL   PHILOSOPHY. 

speed,  it  encounters  not  merely  twice  as  much  resist- 
ance from  the  water,  but  four  times  as  much.  This 
makes  it  much  more  expensive  to  move  boats  rapidly 
than  slowly,  for  it  would  require  -nine  times  the  force 
to  triple  the  speed. 


CHAPTER  III. 
HYDROSTATICS. 

PRESSURE     OF     FLUIDS SPECIFIC     GRAVITY MOTION    OF    FLUIDS — 

WONDERFUL  PROPERTIES  COMBINED  IN  WATER. 

62.  HYDROSTATICS  is  that  branch  of  Natural  Phi- 
losophy, which  treats  of  the  pressure  and  motion  of  fluids 
in  the  form  of  water. 

SEC.  1.     Of  the  PRESSURE  of  Fluids. 

63.  Water,  on  account  of  the  mobility  of  its  parts, 
may  be  easily  displaced,  but  it  is  with  great  difficulty 
compressed.     If  we  take   a   hollow   ball   of  even   so 
compact  a  metal  as  gold,  fill  it  full  of  wTater,  plug  it 
close,  and  put  it  into  a  vise  and  compress  it,  the  water 
will  sooner  force  its  way  through  the  gold  than  yield 
to  the  pressure.     This  is  an  old  experiment,  and  it  led 
to  the  belief  that  water  is  wholly  incompressible  ;  but 
it  is   now  found   that  its  volume  may  be  reduced  to 
smaller  dimensions  by  subjecting  it  to  very  great  pres- 
sures.    Thus,  30,000  pounds  pressure  to  the  inch  will 
lessen  its  bulk  one  twelfth. 

64.  A  fluid  when  at  rest,  presses  equally  in  all  direc- 
tions.    A  point  in  a  tumbler  of  water,  for  example, 
taken   at   any  depth,    exerts   and   sustains  the  same 
pressure  in  all  directions,    upward,    downward,    and 
sidewise.     So  that  if  I   attach  a  string  to  a  musket 

62.  Define  Hydrostatics.   63.  Is  water  compressible  ?  Experiment. 
What  force  is  required  to  lessen  its  bulk  one  twelfth  ? 
64.  What  is  the  law  of  pressure  in  all  directions  *?    Example. 


HYDROSTATICS.  65 

ball  and  let  it  down  into  water,  the  weight  of  the  water 
which  rests  on  its  upper  side  is  balanced  by  an  equal 
pressure  on  its  under  side.  This  is  the  most  remark- 
able property  of  fluids,  and  is  what  distinguishes  them 
from  solids,  which  press  only  downward,  or  in  the  di- 
rection of  gravity. 

65.  A  given  pressure,  or  Now,  impressed  on  any 
portion  of  a  mass  of  water  confined  in  a  vessel,  is  dis- 
tributed equally  through  all  parts  of  the  mass.  If  I 
thrust  a  cork  into  a  bottle  filled  with  water,  so  near 
the  top  that  the  cork  meets  it,  the  pressure  is  felt,  not 
merely  in  the  direction  of  the  cork,  or  just  under  it, 
but  on  all  parts  of  the  bottle  alike ;  and  the  bottle  is 
as  likely  to  break  in  one  part  as  another,  if  equally 
strong  throughout,  and  if  not  equally  strong,  it  will 
give  way  at  its  weakest  point,  wherever  that  is  situ- 
ated. If  we  insert  into  a  large  vessel  of  water  a 
blown  bladder,  and  then  press  upon 
the  upper  surface  of  the  water  with  Fig.  33. 
a  lid  that  fits  it  close,  as  in  figure  33, 
the  bladder  will  indicate  an  equal 
pressure  on  all  sides.  A  is  the  lid 
that  fits  the  jar,  water-tight,  and  is 
applied  to  the  top  of  the  fluid ;  B  is 
a  small  blown  bladder,  kept  in  its 
place  by  a  leaden  weight  resting  on 
the  bottom  of  the  vessel.  If  a  thin 
glass  ball  is  substituted  for  the  blad- 
der, on  pressing  down  the  lid,  it  will 
be  broken  into  minute  fragments,  showing  an  equal 
pressure  on  all  sides.  The  same  effects  would  follow 
were  the  pressure  applied  at  the  side,  or  any  other 
part  of  the  vessel,  instead  of  the  top. 

66.  This  principle  operates  with  astonishing  power 
in  the  hydrostatic  press.     Figure  34  represents  a  press 

65.  How  is  a  pressure  on  any  part  of  a  confined  mass  of  water 
distributed  ?    Example.    Explain  Figure  33. 
6* 


66 


NATURAL  PHILOSOPHY. 


made  of  a  strong  frame  of  timbers,   having  a  large 
cylinder,  C  D,  full  of  water, 


Fig.  34. 


B 


and  opening  into  a  small 
cylinder,  A  B,  in  which  a 
plug  (called  a  piston)  is 
moved  up  and  down  by  the 
lever  attached  to  it.  At 
D  is  another  piston,  which 
when  forced  upward  press- 
es upon  a  follower  at  E, 
which  communicates  the 
force  to  a  pile  of  books  sup- 
posed in  the  process  of  bind- 
my  hand  to  the  lever  and 
AB  upon  the  surface  of 
force  it  presses  upon  the 


ing.     Now   if  I   apply 
force  down   the   piston  in 
the  water,   with  whatever 

surface  of  the  fluid  in  the  small  cylinder,  the  same 
is  exerted  on  all  parts  of  the  water  in  the  large  cylin- 
der, and  consequently  upon  the  piston  D  to  push  it 
upward  against  E.  Suppose  the  number  of  square 
inches  in  the  bottom  of  the  piston  E,  is  a  thousand 
times  as  great  as  in  that  of  the  piston  at  B ;  then  by 
urging  B  forward  with  a  force  equal  to  one  hundred 
pounds,  I  should  communicate  to  E  a  pressure  of  one 
hundred  thousand  pounds.  The  water  in  the  small 
cylinder  would  descend  a  thousand  times  as  much  as 
that  in  the  large  cylinder  rose,  so  that  the  space 
through  which  the  accumulated  force  could  act  would 
be  very  small ;  still  it  would  be  sufficient  for  such 
articles  as  books,  where  the  whole  compression  is  but 
small.  Since  there  is  no  loss  from  friction  in  this 
machine,  a  man  can  by  means  of  it  exert  a  greater 
power  than  by  any  other  to  which  he  can  apply  his 
own  strength.  He  can  by  means  of  it  crush  rjocks, 

66.  Describe  the  Hydrostatic  Press.  Suppose  the  number  of 
square  inches  in  the  larger  piston  is  a  thousand  times  as  great  as 
in  the  smaller  1  Uses  of  the  Hydrostatic  Press  1 


HYDROSTATICS. 


67 


and  cut  in  two  the  largest  bars  of  iron.  The  hydro- 
static press  is  much  used  as  an  oil  press,  as  in  ex- 
tracting oil  from  flaxseed ;  and  also  for  packing  hay, 
cotton,  and  other  light  substances. 

67.  The  surface  of  a  fluid  at  rest  is  horizontal. 
This  property  is  applied  to  the  construction  of  the 
FLUID  LEVEL,  used  by  carpenters,  masons,  and  othe 

L 

Fig.  35. 


workmen.  It  usually  consists  of  a  flat  rule,  having  a 
horizontal  glass  tube  on  the  upper  side,  containing 
alcohol,  (which  is  preferred  to  water  because  it  never 
freezes.)  The  tube  is  not  quite  full  of  the  fluid,  so 
that  when  laid  on  its  side  a  bubble  of  air  floats  on  the 
upper  surface.  When  this  is  exactly  at  a  given  mark 
near  the  middle,  then  the  surface  on  which  the  rule 
is  laid  is  level.  Figure  -p.  36> 

36  represents  a  levelling 
staff  much  used  in  sur-  •*• 
veying  and  grading 
lands.  The  liquid  in  the 
two  arms  of  the  tube  at 
A  and  B  being  precisely 
on  a  level,  any  two  re- 
mote objects,  P  and  Q, 
may  be  brought  accu- 
rately to  the  same  level 
by  sighting  P  with  the 
eye  at  A  ;  that  is,  bring- 
ing it  into  the  same  hori- 
zontal line  with  the  sur-  ~~" 
faces  of  A  and  B,  and 
then  sighting  Q  in  the  same  manner. 

67.  How  is  the  surface  of  a  fluid  when  at  rest  1    Describe  the 
fluid-level  and  the  levelling  staff. 


68 


NATURAL   PHILOSOPHY. 


68.  The  pressure  upon  any  portion  of  a  column  of 
fluid,  is  proportioned  to  its  depth  below  t\e  surface.  If 
we  let  down  a  junk  bottle  into  the  sea,  the  pressure 
on  all  sides  of  it  would  continually  increase  as  it  de- 
scended, until  it  would  be  sufficient  to  crush  it.  Its 
great  strength,  however,  would  enable  it  to  bear  a  pro- 
digious pressure.  When  an  empty  bottle,  corked 
closely,  is  let  down  to  a  great  depth,  on  drawing  it 
up,  it  is  found  full  of  salt  water,  and  yet  the  cork  un- 
disturbed. At  a  certain  depth,  the  pressure  on  the 
cork  is  such  as  to  contract  its  dimensions,  and  yet, 
being  equally  pressed  on  all  sides,  it  is  not  displaced. 
Its  size  being  contracted,  the  water  runs  in  at  the 
sides;  but  on  rising  to  the  surface,  the  cork  swells 
again  to  its  former  bulk.  When  the  stopper  does  not 
admit  of  compression,  the  water  sometimes  is  forced 
through  its  pores,  and  thus  fills  the  bottle.  Ships  sunk 
Fig,  37.  at  9-  great  depth,  have  their  wood  ren- 
\  dered  so  heavy  by  the  great  quantity  of 
water  forced  into  it,  that  when  they  go 
to  pieces  their  parts  do  not  rise.  The 
pressure  of  water  on  a  square  foot,  at  the 
depth  of  eight  feet,  is  500  pounds ;  and 
having  the  same  amount  added  for  every 
[  eight  feet  of  descent,  it  soon  becomes 
II  prodigious.  At  the  depth  of  a  mile,  it 
is  no  less  than  330,000  pounds  upon  the 
square  foot. 

69.  Fluids  rise  to  the  same  level  in  the 
opposite  arms  of  a  bent  tube.  Let  Fig.  37 
be  a  bent  tube :  if  water  be  poured  into 
either  arm  of  the  tube,  it  will  rise  to  the 
)  same  height  in  the  other  arm.  Nor  is  it 
material  what  may  be  the  shape,  size,  or 

68.  How  is  the  pressure  of  a  column  of  fluid  at  different  depths? 
Example  in  a  junk-bottle.  What  happens  to  a  corked  bottle  sunk 
to  a  great  depth  1  What  is  the  pressure  on  a  square  foot  at  the 
depth  of  eight  feet — and  a  mile  ? 


HYDROSTATICS.  69 

inclination  of  the  opposite  arms.     Figure  38  represents 
a  variety  of  vessels  and  tubes  open  at  top,  but  corn- 
Fig.  38. 


municating  with  a  common  cistern  of  water  below. 
If  we  pour  water  into  any  one  of  these,  so  as  to  fill  it 
to  any  height,  the  water  will  be  at  the  same  height 
in  each  of  the  others.  Hence,  water  conveyed  in 
aqueducts,  or  running  in  natural  confined  channels,  will 
rise  just  as  high  as  its  source,  and  no  higher.  Be- 
tween the  place  of  exit  and  the  spring,  the  ground  may 
rise  into  hills  and  descend  into  valleys,  and  the  pipes 
which  convey  the  water  may  follow  all  the  irregulari- 
ties of  the  country,  and  still  the  water  will  run  freely, 
provided  no  pipe  is  laid  higher  than  the  level  of  the 
spring. 

70.  The  pressure  of  a  column  of  water  upon  the  lot- 
torn  of  a  vessel,  depends  wholly  upon  the  height  of  the 
column,  without  regard  to  its  shape  or  size.  In  Fig. 
38  the  pressure  on  the  bottom  of  the  cistern  will  be 
the  same,  whether  one  tube  is  attached,  or  the  whole 
number,  or  the  vessel  itself  is  raised  to  the  same 
height  all  the  way  of  the  same  size  as  at  the  bottom, 

69.  Fluids  in  the  opposite  arms  of  a  bent  tube  ?    What  does  Fig. 
38  represent  1    How  high  will  water  in  an  aqueduct  rise  1 

70.  Upon  what  does  the  pressure  of  a  column  of  fluid  on  the  bottom 


70 


NATURAL    PHILOSOPHY. 


Fig.  39. 


or  even  if  swelled  out  like  a  funnel,  so  as  to  be  much 
larger  above  than  below.  On  this  principle  is  founded 
the  hydrostatic  paradox — that  any  quantity  of  water 
however  small  may  be  made  to  raise  any 
weight  however  great.  Fig.  39  repre- 
sents a  bellows  having  on  one  side 
an  open  tube  communicating  with  it. 
On  pouring  water  into  the  tube  (the 
bellows  being  full)  it  will  force  up  the 
top  of  the  bellows,  although  loaded  with 
heavy  weights.  A  wine-glass  of  water, 
for  example,  will  raise  the  boys  that 
stand  on  the  bellows,  and  would  sensi- 
bly lift  a  weight  many  hundred  time? 
as  great.  The  principle  is  the  same 
as  in  the  hydrostatic  press.  Here  the 
weight  of  the  column  of  water  affords 
the  power  that  acts  on  the  larger  end  of  the  bellows, 
as  in  the  press  the  force  of  the  piston  in  the  small 
cylinder  acts  on  that  in  the  larger. 

SEC.  2.     Of  Specific  Gravity. 

71.  SPECIFIC  GRAVITY  is  the  weight  of  a  body  com- 
pared with  another  of  the  same  bulk,  taken  as  a  stand- 
ard. Water  is  the  standard  for  solids  and  liquids; 
common  air  for  gases.  The  specific  gravity  of  a 
mineral,  for  example,  or  of  alcohol,  is  its  weight  com- 
pared  with  that  of  a  mass  of  water  of  exactly  the 
same  volume ;  the  specific  gravity  of  steam  is  its 
weight  compared  with  that  of  the  same  volume  of  at- 
mospheric air.  We  must  know,  then,  what  an  equal 
volume  of  the  standard  would  weigh.  This  is  ascer- 

of  a  vessel  depend  1    State  the  principle  called  the  hydrostatic  para- 
dox.   Explain  Fig.  39. 

71.  Define  specific,  gravity.    What  is  the  standard  for  solids — what 
for  liquids — what  for  gases  ?  What  must  we  knoiv  in  order  to  find  the 


HYDROSTATICS. 


71 


Fig  40. 


tained  in  the  case  of  a  solid,  by  finding  how  much 
less  the  body  weighs  in  water  than  in  air;  and,  in  the 
case  of  a  liquid  or  a  gas,  by  weighing  equal  volumes 
of  the  body  and  of  air.  Wishing  to  know  how  much 
heavier  a  certain  ore,  which  I  suspected  to  be  silver, 
was  than  water,  I  tried  to  compare  its  weight  with 
that  of  an  equal  bulk  of  water ;  but  the  ore  being  of 
very  irregular  shape,  I  found  great  difficulty  in  meas- 
uring it  accurately  to  find  the  number  of  solid  inches 
in  it,  so  that  I  could  weigh  it  against  the  same  num- 
ber of  inches  of  water.  But  learning  that  a  body 
when  weighed  in  water  weighs  as  much  less  than 
when  weighed  in  air,  as  is  just  equal  to  the  weight 
of  the  same  volume  of  water,  I  attached  a  string  to 
the  ore,  hung  it  to  one  arm  of  the  balance,  and  found 
its  weight  to  be  4.75  ounces; 
and  then  bringing  a  tumbler  of 
water  under  the  suspended  ore 
so  as  to  immerse  it,  1  found  it 
did  not  in  this  situation  weigh 
as  much  as  before,  but  I  had  to 
take  out  1.25  ounces  to  restore 
the  balance.  This,  then,  was 
what  the  ore  lost  in  water,  and 
was  the  weight  of  an  equal 
volume  of  water.  Now  I  have 
found  that  the  ore  weighs  four 
ounces  and  three  quarters,  while 
the  same  bulk  of  water  weighs 
only  one  ounce  and  a  quar- 
ter. I  see,  therefore,  at  once,  that  the  ore  is  about 
four  times  as  heavy  as  water ;  but  to  find  the  exact 
specific  gravity,  I  see  how  many  times  the  weight  of 
the  ore  is  greater  than  that  of  an  equal  volume  of 
water,  by  dividing  4. "7  5  by  1.25,  which  gives  3.8  as 

specific  gravity  of  a  body  1    Describe  the  way  of  finding  the  speci- 
fic gravity  of  an  ore — also  of"  alcohol — also  of  carbonic  acid. 


72  NATURAL  PHILOSOPHY. 

the  exact  specific  gravity  of  the  ore.  I  conclude, 
therefore,  that  it  cannot  contain  much  silver,  if  any ; 
otherwise  it.  would  be  heavier.  Again,  desiring  to 
find  the  specific  gravity  of  some  alcohol,  (which  is 
better  in  proportion  as  it  is  lighter,)  I  took  a  small 
vial,  counterpoised  it  in  a  pair  of  delicate  scales, 
and  poured  in  water  gradually  till  I  had  introduced 
exactly  1000  grains.  I  then  set  the  vial  on  the 
table,  and  placing  my  eye  accurately  on  a  level  with 
the  surface  of  the  water,  I  made  a  fine  mark  with  a 
small  file  just  round  the  water  line.  On  emptying  out 
the  water  and  filling  the  vial  to  the  same  mark  with 
the  alcohol,  I  found  the  weight  of  it  to  be  815  grains. 
I  therefore  inferred  that  its  specific  gravity  was  815, 
water  being  1000.  Having  now  my  vial  ready,  I 
filled  it  to  the  mark  successively  with  half  a  dozen 
different  liquors,  some  lighter  and  some  heavier  than 
water,  and  thus  found  the  exact  specific  gravity  of  each. 
Finally,  I  had  the  curiosity  to  see  which  is  the  heav- 
iest, common  air,  or  that  sort  of  air  which  sparkles  so 
briskly  in  soda-water,  and  in  bottled  beer,  called  car- 
bonic acid.  I  therefore  weighed  a  light  glass  bottle, 
which,  as  we  commonly  say,  was  empty,  but  was  really 
filled  with  common  air,  and  then  withdrawing  the  air 
from  the  bottle  by  means  of  a  kind  of  syringe  which 
sucked  it  all  out,  I  then  turned  the  stop-cock  attached 
to  the  mouth,  shut  the  bottle  close,  and  weighing  it 
again,  found  it  had  lost  40  grains,  which  was  the  weight 
of  the  air.  At  last  I  filled  the  bottle  with  carbonic  acid 
instead  of  air,  and  weighing  again,  found  the  vessel  now 
weighed  60  grains  more  than  before.  This  was  the 
weight  of  the  carbonic  acid ;  and  now  having  found 
that  when  we  take  equal  bulks  of  common  air  and 
carbonic  acid,  the  latter  weighs  60  grains,  while  the 
former  weighs  only  40,  I  infer  that  the  carbonic  acid 
is  one  half  heavier  than  common  air ;  that  is,  its  spe- 
cific gravity  is  1.5.  By  a  similar  process,  I  found 


HYDROSTATICS.  73 

that  hydrogen  gas,  one  of  the  elements  of  water,  is 
more  than  thirteen  times  as  light  as  air,  being  the 
lightest  of  all  known  bodies. 

72.  A  body  floats  in  water  at  any  depth,  when  its 
specific  gravity  is  just  equal  to  that  of  water.  The 
human  system  is  a  little  heavier  than  water,  and  there- 
fore tends  to  sink  in  it ;  but  if  we  strike  the  water 
downward,  its  reaction  will  keep  us  up,  acting  as  it 
does  in  a  direction  opposite  to  that  of  gravity.  A  very 
slight  blow  upon  the  water  is  sufficient  to  balance  the 
downward  tendency,  and  therefore  swimming  becomes 
an  easy  matter  when  skillfully  practised.  As  we  lose 
in  water  as  much  of  our  weight  as  the  same  bulk  of 
water  would  weigh,  and  that  is  nearly  the  whole,  it  is 
only  the  slight  excess  of  our  weight  which  we  have 
to  sustain  in  swimming.  Indeed,  if  we  could  keep 
our  lungs  constantly  inflated,  we  should  require  no  re- 
action to  keep  us  up,  but  should  float  on  the'  surface. 
Dr.  Franklin  when  a  boy  swam  across  a  river  by  the 
aid  of  his  kite,  which  supplied  the  upward  force  neces- 
sary to  sustain  him,  instead  of  the  reaction  of  the  wa- 
ter. Fishes  are  nearly  of  the  same  specific  gravity 
as  the  water  in  which  they  live.  They  are  supplied 
with  a  small  air-bladder,  which  they  have  the  power 
of  compressing  and  dilating.  When  they  wish  to  sink 
they  compress  this  bladder,  and  their  specific  gravity- 
is  then  greater  than  that  of  the  water  ;  and  they  easi- 
ly rise  again  by  suffering  the  bladder  to  dilate.  Birds 
float  in  the  atmosphere  on  similar  principles.  Being 
but  little  heavier,  bulk  for  bulk,  than  the  air,  very 
slight  blows  with  their  wings  create  the  reaction  in 
an  upward  direction,  which  is  necessary  to  sustain 
them ;  stronger  blows  cause  the  reaction  to  overbalance 


72.  When  does  a  body  float  in  water "?    How  is  the  body  sup- 
ported in  simmming  1    How  did  Dr.  Franklin  swim  across  a  river  7 
How  do  fishes  ascend  and  descend  1    How  do  birds  fly  1 
7 


74  NATURAL    PHILOSOPHY. 

the  excess  of  their  specific  gravity  over  that  of  the  air, 
and  they  rise  with  the  difference. 

73.  When  a  body  floats  on  the  surface  of  water,  it 
displaces  as  much  weight  of  water  as  is  equal  to  its 
own  weight.  Thus,  if  I  place  a  wooden  block  weigh- 
ing four  ounces  in  a  tumbler  of  water  even  full,  just 
four  ounces  of  the  water  will  run  over,  as  we  may 
ascertain  by  collecting  and  weighing  it.  Upon  this 
principle  ships  float  on  water.  In  proportion  as  we 
lade  the  ship,  it  sinks  deeper  and  deeper,  the  weight 
of  water  displaced  always  being  exactly  equal  to  the 
weight  of  the  ship  and  cargo.  The  actual  weight  of 
the  ship  and  cargo  may  be  easily  ascertained  on  this 
principle ;  for  if  we  float  the  ship  into  a  dock  of  known 
size,  containing  a  given  quantity  of  water,  the  weight 
of  the  ship  and  cargo  may  be  determined  from  the  rise 
of  the  water,  in  the  dock.  A  boy  wished  to  find  the 
tonnage  of  his  boat.  He 
£-  41*  therefore  loaded  it  as  heavy 

as  it  would  swim,  and  then 
transferred  it  to  a  small  box 
which  he  had  made,  and  of 
which  he  knew  the  exact 
[dimensions.  He  then  poured 
!  into  the  box  a  pound  of  wa- 
ter at  a  time,  and  when  it 
Jhad  settled  to  a  good  level, 
he  made  a  mark  at  the  water  line,  arid  adding  one 
pound  of  water  at  a  time,  he  thus1  had  marks  at  differ- 
ent  heights,  from  one  pound  up  to  twenty.  He  found 
that  four  pounds  of  water  were  amply  sufficient  to 
float  his  boat,  and  when  the  boat  was  laid  upon  it, 
the  water  rose  on  the  sides  to  the  nineteenth  mark. 
Consequently  the  boat  had  raised  the  water  fifteen 

73.  How  much  water  does  a  floating  body  displace  ?  Example 
Method  of  finding  the  tonnage  of  a  ship?  How  did  the  boy  find  the 
tonnage  of  his  boat  1 


HYDROSTATICS.  75 

marks,  and  its  weight  was  of  course  fifteen  pounds ; 
for  it  weighed  just  as  much  as  the  water  would  have 
weighed  which  it  would  have  taken  to  raise  the  level 
from  the  fourth  to  the  nineteenth  mark. 

SEC.  3.     Of  the  MOTION  of  Fluids. 

74.  That  part  of  hydrostatics  which  treats  of  the 
mechanical  properties  and  agencies  of  running  water, 
is  called  Hydraulics,  and  machines  carried  by  water, 
or  used  for  raising  it,  Hydraulic  machines.  It  em- 
braces  what  relates  to  water  flowing  in  open  channels, 
as  rivers  and  canals ;  or  in  pipes,  as  aqueducts ;  or 
issuing  from  reservoirs  in  jets  and  fountains ;  or  falling, 
as  in  dams  and  cascades  ;  or  oscillating  in  waves. 
A  river  or  canal  is  water  rolling  down  hill,  and  would 
be  subject  to  the  same  law  as  other  bodies  descending 
inclined  planes,  were  it  not  for  the  numerous  impedi- 
ments which  oppose  the  full  operation  of  the  law. 
Now  a  body  rolling  down  an  inclined  plane  has  its 
motion  constantly  accelerated,  like  a  body  falling  per- 
pendicularly, gaining  the  same  speed  in  descending 
the  plane  that  it  would  in  falling  through  the  perpen- 
dicular height  of  the  plane.  Hence  when  a  body  rolls 
down  a  long  plane  without  obstruction,  it  soon  acquires 
an  immense  velocity,  as  is  seen  in  a  rock  rolling  down 
a  long  hill.  In  the  same  manner,  a  body  of  water 
descending  in  a  river  constantly  tends  to  run  faster 
'and  faster,  and  would  soon  acquire  a  most  destructive 
I  momentum,  were  it  not  retarded  by  numerous  coun- 
jteracting  causes,  the  chief  of  which  are  the  friction  of 
;the  banks  and  bottom,  and  the  resistance  occasioned 
V  its  winding  course,  every  turn  opposing  an  impedi- 
ment of  more  or  less  force.  By  such  a  circuitous 
route  two  benefits  are  gained — the  rapidity  of  the 

74.  Define  Hydraulics.   What  subjects  does  it  embrace.  1  Are  rivers 
subject  to  the  laws  of  falling  bodies  1    What  benefits  arise  from 


76  NATURAL   PHILOSOPHY. 

stream  is  checked,  and  its  advantages  are  more  widely 
distributed.  A  river  flows  faster  in  the  channel,  to- 
wards the  middle,  than  near  the  banks,  because  it  is 
less  retarded  by  friction ;  and  during  a  freshet  the 
rapidity  is  greatly  increased,  because  since  the  waters 
that  are  piled  on  the  original  bed  are  subject  to  little 
friction,  they  exhibit  something  of  the  accelerated  mo- 
tion of  bodies  rolling  freely  down  inclined  planes.  A 
very  slight  fall  is  sufficient  to  give  motion  to  water 
where  the  impediments  are  slight.  The  Croton  Aque- 
duct, that  waters  the  city  of  New  York,  falls  but  one 
foot  in  a  mile.  Three  feet  fall  per  mile  makes  a 
mountain  torrent.  Some  rivers  do  not  fall  more  than 
500  feet  in  1000  miles,  or  a  foot  in  two  miles,  and  re- 
quire a  number  of  days,  or  even  weeks,  to  pass  over 
this  distance. 

75.  The  Aqueducts  which  the  ancient  Romans 
and  Carthaginians  built  for  watering  their  cities, 
were  among  the  greatest  of  their  works,  some  of 
which  have  remained  until  the  present  day.  Large 
streams  were  conducted  for  many  miles,  sometimes 
not  less  than  a  hundred,  in  open  canals,  carried  through 
mountains  and  led  over  deep  valleys,  on  stupendous 
arches  of  masonry.  Some  have  supposed  that  the 
ancients  must  have  been  unacquainted  with  the  prin- 
ciple, that  water  flowing  in  pipes  will  rise  as  high  as 
its  source,  since,  had  they  known  this,  they  might 
have  conveyed  water  in  pipes  instead  of  such  expen- 
sive structures ;  these  might  have  ascended  and  de- 
scended, following  all  the  inequalities  of  the  face  of 
the  country,  provided  they  were  in  no  part  higher 
than  the  head  or  spring.  It  is  found,  however,  that 
they  were  acquainted  with  the  principle,  but  prefer- 

the  circuitous  routes  of  rivers  1    What  part  of  a  river  flows  fastest  ? 
Why  do  rivers  run  so  swift  during  a  freshet  1    What  fall  per  mile 
have  the  Croton  Water  Works  1 
75,  What  of  the  Aqueducts  of  the  Romans  and  Carthaginians  * 


HYDROSTATICS.  77 

red  to  construct  their  aqueducts  of  open  channels  rath- 
er than  pipes.  Suitable  pipes,  at  that  age,  would 
have  been  very  costly.  They  are  apt  also  to  become 
clogged  ;  and  although  they  might  have  followed  the  in- 
equalities of  hills  and  valleys,  yet  when  they  descend- 
ed and  ascended  fa,r  from  the  general  level,  they  would 
be  obliged  to  encounter  an  enormous  pressure,  since 
in  a  column  of  water,  the  pressure  on  any  part  is  proi< 
portioned  to  the  depth  below  the  surface  of  the  water, 
increasing  five  hundred  pounds  to  the  square  foot  for 
every  eight  feet  of  descent.  A  pipe,  therefore,  fifty 
feet  deep  and  full  of  water,  would  have  to  bear  a  pres- 
sure at  the  lower  part  of  more  than  three  thousand 
pounds  to  the  square  foot,  and  must  be  made  propor- 
tionally strong,  and  would  be  apt  to  leak  at  the  joints. 
Even  at  the  present  day,  it  is  found  more  eligible  to 
water  cities  by  open  aqueducts  than  by  pipes,  as  is 
done  in  the  new  Croton  Water  Works  for  watering  the 
city  of  New  York.  Here  an  artificial  river  of  the 
purest  water  is  conveyed  from  the  county  of  Westches- 
ter,  forty-one  miles  above  the  city,  to  a  vast  reservoir 
capable  of  holding  150,000,000  of  gallons,  where  it  has 
opportunity  to  deposit  any  sediment  or  impurities  it 
may  have  taken  up  on  its  way,  and  to  absorb  air,  which 
gives  it  life  and  briskness.  From  the  reservoir  it  is 
distributed  to  all  parts  of  the  city  in  pipes,  affording 
an  ample  supply  for  domestic  uses,  for  watering  and 
washing  the  streets,  and  for  extinguishing  fires. 

76.  When  a  plug  is  removed  from  the  top  of  one  of 
the  pipes  of  an  aqueduct,  the  water  spouts  upward  in 
a  jet ;  for,  since  water  thus  situated  tends  to  rise  as 
high  as  its  source,  it  will  spout  to  that  height  when 
unconfmed.  At  least  it  would  ascend  to  that  height 

Were  the  ancients  acquainted  with  the  principle  that  water  as- 
cends to  the  level  of  its  source  1   Describe  the  Croton  Water  Works. 
76.  Why  does  water  spout  from  a  pipe  of  an  aqueduct  1    How 
high  will  it  spout  ? 

7* 


78  NATURAL   PHILOSOPHY. 

were  it  not  for  the  resistance  of  the  air,  which  pre- 
vents its  attaining  that  full  height.  It  is  on  this  prin- 
ciple that  fountains  are  constructed.  If  we  open  a 
vent  in  the  side  of  a  water-pipe,  so  as  to  let  the  jet 
out  obliquely,  it  will  form  the  curve  of  a  parabola ; 
and  by  letting  out  the  jet  through  different  orifices, 
the  curves  may  be  varied,  and  beautiful  and  pleasing 
figures  exhibited,  as  is  shown  at  the  Park  Fountain 
in  the  city  of  New  York. 

77.  In  building  tall  or  deep  cisterns,  we  must  re- 
member, that  the  pressure  on  any  part  of  the  cistern 
increases  with  the  depth,  and  hence  that  the  lower 
parts  require  to  be  made  stronger  and  closer  than  the 
upper,  else  they  will  either  burst  in  pieces  or  leak. 
A  philosopher  wishing  to  provide  a  constant  supply 
of  water  near  his  house,  constructed  a  large  cistern 
six  feet  high,  and  contrived  to  convey  a  small  stream 
of  water  to  the  top  which  kept  it  always  full  and  run- 
ning over  by  a  waste-pipe.  In  the  side  of  the  cistern 
he  inserted  two  large  stop-cocks  of  equal  size,  the 
first,  one  foot,  and  the  other  four  feet  from  the  top, 
supposing  that  he  might,  in  a  given  time,  draw  off  either 
one  gallon  or  four  gallons ;  but  he  was  surprised  to  find 
that  he  could  obtain  from  the  lower  stop-cock  only 
twice  as  much  as  from  the  upper.  How,  thought  he, 
is  this  consistent  with  the  principle  that  the  pressure 
is  proportioned  to  the  depth  ?  If  it  presses  against 
the  side  of  the  cistern  at  the  lower  level  four  times  as 
much  as  at  the  upper,  why  do  not  four  times  as  many 
gallons  run  out  when  the  stopper  is  opened  ?  On  re- 
flection, however,  he  perceived  that  the  pressure  on 
the  side  must  be  proportioned  to  the  momentum,  which 
depends  on  two  things — the  quantity  of  matter  and 
the  velocity ;  and  of  course  that  twice  the  quantity  of 

77.  How  must  we  provide  for  the  strength  of  a  pipe  at  different 
heights'?  Relate  the  story  of  the  philosopher  drawing  water  from  a 
cistern.  To  what  is  the  quantity  of  water  discharged  from  a  cistern  at 


HYDROSTATICS.  79 

water  flowing  with  twice  the  velocity,  would  have 
just  four  times  the  momentum.  Hence  he  learned  the 
grand  principle,  that  in  a  column  of  water  kept  con- 
stantly full,  the  quantity  discharged  from  any  orifice 
in  the  side,  is  proportioned  to  the  square  root  of  the 
depth  below  the  surface  of  the  fluid.  So  that,  to  draw 
off  twice  as  much,  we  must  make  the  opening  four 
times  as  deep,  and  to  draw  off  three  times  as  much,  we 
must  make  it  nine  times  as  deep. 

78.  The  philosopher  tried  another  experiment  with 
his  cistern.     He  turned  off  the  run  of  water  that  sup- 
plied  the  cistern,  and  then   opened   the    upper    stop- 
cock, and  found  it  took  just  five  minutes  to  draw  off 
the  water  to  that  depth.     He  then  let  in  the  run  that 
supplied  the  cistern  and  kept  it  constantly  full.     Now 
opening  the  same  orifice  again,  and  drawing  off  for  five 
minutes  more,  he  found  that  he  caught  just  twice  as 
much  water  as  before.     From  this  he  inferred,  that  if 
a  vessel  discharges  a  certain  quantity  of  water  in  emp- 
tying itself  to  a  certain  level,  it  will  discharge  twice 
as  much  in  the  same  time,  when  the  vessel  is  keot 
constantly  full. 

79.  Water  issues  from  the  bottom  or  side  of  a  ves- 
sel with  the  same  force  that  it  would  acquire  by  fall- 
ing through  the  perpendicular  height  of  the  column. 
It  would  therefore  seem  to  make  no  difference  whether 
we  let  water  fall  upon  a  water-wheel  from  the  top  of 
a  cistern,  or  whether  we  raise  a  gate  at  the  bottom 
of  the  column,  and  let  the  water  issue  so  as  to  strike 

•  the  wheel  there,  since  it  would  strike  the  wheel 
in  both  cases  with  the  same  velocity,  except  what 
might  be  lost  in  the  falling  column  by  the  resistance 

different  depths  proportioned  1    How  much  lower  must  we  go  to 
double  the  quantity  1 

78.  What  other  experiment  did  he  try  1    How  much  more  is  dis- 
charged when  the  vessel  is  kept  constantly  full  1 

79.  With  what/orce  does  water  issue  from  the  bottom  or  side  of  a 
vessel  1    Does  it  make  any  difference  whether  water  falls  upon  a 


80  NATURAL  PHILOSOPHY. 

of  the  air.  A  waterfall  like  that  of  Niagara,  where 
an  immense  body  of  water  rolls  first  in  rapids  down  a 
long  inclined  plane,  and  then  descends  perpendicu- 
larly from  a  great  height,  affords  one  of  the  greatest 
exhibitions  of  mechanical  power  ever  seen.  The  Falls 
of  Niagara  contain  power  enough  to  turn  all  the  mills 
and  machinery  in  the  world.  They  waste  a  greater 
amount  of  power  every  minute,  than  was  expended  in 
building  the  pyramids  of  Egypt ;  for,  in  that  short 
space  of  time,  millions  of  pounds  of  water  go  over  the 
falls,  and  each  pound,  by  the  velocity  it  gains  in  fall- 
ing first  down  the  rapids,  and  then  perpendicularly, 
acquires  resistless  energy.  Water  falling  one  hundred 
feet  would  strike  on  every  square  foot  with  a  force  of 
more  than  six  thousand  pounds. 

80.  Man  imitates  the  power  of  the  natural  water- 
fall when  he  builds  a  dam  across  a  stream,  raising  it 
above  its  natural  level,  and  then  turning  aside  more 
or  less  of  it  into  a  narrow  channel,  makes  it  acquire 
momentum  while  regaining  its  original  level.  When 
it  has  gained  the  requisite  force,  he  turns  it  upon  a 
water-wheel  usually  of  great  size,  from  which,  by 
means  of  machinery,  the  force  is  distributed  wherever 
it  is  wanted,  and  so  applied  as  to  do  all  sorts  of  work. 
When  a  run  of  water  first  strikes  a  wheel  at  rest,  it 
strikes  it  with  its  full  force ;  but  as  the  wheel  moves 
before  it,  the  effect  of  the  force  is  diminished,  and  if 
the  wheel  acquired  the  same  velocity  as  the  stream, 
the  force  would  become  nothing.  The  wheel  is  re- 
tarded by  making  it  do  more  and  more  work,  or  carry 
a  greater  weight,  until  it  acquires  a  uniform  motion  at 
a  certain  rate,  which  ought  to  be  that  at  which  the  force 
of  the  stream  produces  the  greatest  effect.  This  is 

wheel  from  the  top,  or  issues  upon  it  from  the  bottom  1  What  of  the 
Falls  of  Niagara  1 

80.  When  does  man  imitate  the  waterfall!  With  what  force  does  a 
run  of  water ,/irs?  strike  a  wheel  1  How  when  the  wheel  is  in  motion  1 


HYDROSTATICS. 


81 


in  some  cases  when  the  wheel  moves  half  as  fast  as 
the  stream.     That  a  current  of  water  or  of  wind  strikes 
an  object  with  less  force  when  the  object  is  moving  the 
same  way,  is  a  general  principle.    Thus,  when  a  steam- 
boat is  moving  directly  before  the  wind,  she  would  de- 
rive little  aid  from  sails  unless  the  wind  were  high, 
for  she  would  "  run  away  from  the  breeze ;"  that  is, 
the  wind  would  produce  no  effect  any  farther  than  its 
velocity  exceeded  that  of  the  boat,  and  if  it  were  just 
equal  to  that,  the  effect  would  be  absolutely  nothing. 
A  man  in  a  balloon,  carried  forward  by  a  wind  blow- 
ing a  hundred  miles  an  hour,  would  speedily  acquire 
the  same  velocity  with  the  wind,  and  therefore  appear 
to  himself  to  be  all  the  while  in  a  calm.     Although 
the  earth  is  constantly  revolving  round  the  sun  with 
inconceivable  rapidity,  yet  as  we  have  the  same  ve- 
locity we  seem  to  be  at  rest. 

SEC.  4.     Of  the  Remarkable  Properties  combined  in 
Water. 

81.  Water  combines  in  itself  a  variety  of   useful 
properties,  all  designed  for  the  benefit  of  man.     First, 
Natural  History  leads  us  to  contemplate  it  in  its  va- 
rious aspects.     It  covers   about  three  fourths  of  the 
globe,  and  is  distributed  into  oceans,  seas,  and  lakes, 
rivers,  springs,  and  atmospheric  vapor.    By  the  agency 
of  heat,  water  is  constantly  rising  in  vapor  on  all  parts 
!    of  the  ocean.     This  mingles  with  the  air  in  an  in- 
visible elastic  state,  being  separated  in  the  process  of 
i    evaporation  from  its    salt   and  every  other  impurity. 
!    More  or  less  of  it  is  conveyed  over  the  land  by  winds, 
•    and  falls  upon  it  in  dew,  and  rain,  and  snow.     A  part 
]    of   this  filters   through  the  sand,  runs    down    in    the 
i  ==rr: 

,     In  what  case  does  the  stream  produce  its  greatest  effect  1    Example 
1     in  a  steamboat.     How  would  a  man  in  a  balloon  appear  to  hirn- 
\     self  to  be  situated  when  moving  with  the  same  velocity  as  the  wind  1 
81.  What_parf  of  the  earth  is  covered  with  water  1     In  what  dif- 
ferent forms '!  What  beneiits  How  from  rivers  ?  Also  from  the  ocean  1 


82  NATURAL  PHILOSOPHY. 

crevices  of  rocks,  and  collects  in  pure  fountains  not  far 
below  the  surface,  where  it  may  be  easily  reached  in 
almost  every  place,  by  digging  wells.  In  various 
places  it  flows  out  by  its  own  pressure,  in  springs  and 
streamlets,  which  unite  in  rivulets,  and  these  in 
rivers,  which  return  the  water  to  the  sea.  But  rivers 
as  they  run  are  made  to  impart  fertility,  and  to  furnish 
an  avenue  by  which  vessels  and  steamboats  may  pene- 
trate into  the  heart  of  every  country,  and  convey  to  the 
remotest  cities  the  riches  of  every  clime.  As  rivers 
furnish  an  entrance  into  the  interior  of  countries,  so 
the  ocean  forms  the  great  highway  between  nations, 
and  unites  all  nations  in  the  bands  of  commerce.  Still 
further,  to  serve  the  grand  cause  of  benevolence,  the 
ocean  is  filled  with  living  beings  innumerable,  which 
are  not,  like  land  animals,  confined  to  the  surface,  but 
occupy  the  depth  of  at  least  six  hundred  feet,  and  thus 
enjoy  a  far  more  extensive  domain  than  the  part  of  the 
animal  creation  that  inherits  the  land. 

82.  Secondly,  Chemistry  regards  water  with  no 
less  interest  than  Natural  History.  Its  very  composi- 
tion is  admirable,  being  constituted  of  two  substances, 
oxygen  and  hydrogen,  which,  when  united  with  heat, 
are  separated  in  the  gaseous  form,  and  each  possesses 
the  most  curious  and  wonderful  properties.  Oxygen 
is  found  as  an  element  in  nearly  all  bodies  in  na- 
ture ;  it  is  the  part  of  atmospheric  air  which  sus- 
tains all  animal  life  and  supports  all  fires  ;  and  it  is 
the  most  active  agent  in  producing  all  the  changes 
of  matter  which  take  place  both  in  nature  and  art. 
Hydrogen  gas  is  the  most  combustible  of  all  bodies, 
and  is  in  fact  what  we  see  burning  in  nearly  every 
sort  of  flame.  As  a  solvent,  water  performs  the  most 
useful  service  to  man,  removing  every  impurity  from 
his  clothing  or  his  person,  dissolving  and  prepar- 

82.  What  is  the  composition  of  water  *  What  of  oxygen  and  hydro- 
gen? What  oi'  water  as  a  solvent  ?   Oi'the  diiierem  states  oi'  water  7 


HYDROSTATICS.  83 

ing  his  food,  and  entering  largely  into  nearly  all  the 
processes  of  the  arts.  By  the  different  states  which 
water  assumes,  of  ice  and  snow  and  vapor,  it  performs 
important  offices  in  the  economy  of  Nature,  as  well 
as  in  its  native  state  of  a  liquid.  These  changes  of 
state  regulate  the  temperature  of  the  atmosphere,  and 
preserve  it  from  dangerous  excesses  both  of  heat  and 
cold.  On  the  one  hand,  on  the  approach  of  winter  in 

|  cold  climates,  water  changes  to  ice  and  gives  out  a 
vast  amount  of  heat  that  kept  it  in  the  liquid  state  ; 
and  on  the  approach  of  summer,  to  check  the  too 
rapid  increase  of  temperature,  the  same  heat  which 
was  given  out  when  water  was  changed  into  ice,  is 
now  absorbed  and  withdrawn  from  the  atmosphere,  as 
ice  is  changed  back  to  water.  Moreover,  during  the 
heat  of  summer,  the  evaporation  of  water,  a  very 
cooling  process,  checks  the  tendency  to  excess  of  the 
heat  of  the  sun,  and  guards  us  from  all  danger  on  that 
hand.  Ice,  by  covering  the  rivers,  keeps  them  from 
freezing  except  on  the  surface  ;  and  snow  is  a  warm 
and  downy  covering  thrown  over  the  earth  to  pro- 
tect the  vegetable  kingdom,  by  confining  the  heat  of 
the  earth. 

83.  Thirdly,  it  is  the  province  of  Physiology  to  con- 
template the  relations  of  water  to  the  vegetable  and 
animal  kingdoms.  Water  is  the  chief  food  of  plants, 
which  it  nourishes,  either  by  supplying  a  part  of  their 
elements,  or  by  dissolving  their  nutriment,  and  thus 
preparing  it  for  circulation  ;  and  hence  water  is  indis- 

;  pensable  to  the  life  and  growth  of  all  vegetables.     To 
animals  and  man,  it  furnishes  the  best  and  only  neces- 

jj  sary  beverage  ;  it  is  the  medium  by  which  our  food  is 

4  prepared  ;   and  it  acts  medicinally  in  various  ways, 

I  both  internally  and  externally. 

How  does  it  check  the  cold  of  winter  and  the  heat  of  summer  1 
Useful  properties  of  ice  and  snow »? 

S3.  What  are  the  relations  of  water  to  the  vegetable  kingdom  1 
,  What  to  animals  and  man  ? 


84  NATURAL   PHILOSOPHY. 

84.  Finally,  the  Mechanical  relations  of  water,  such 
as  those  we  have  been  considering  in  the  preceding 
pages,  are  hardly  less  remarkable  and  important  than 
the  rest.  By  its  mobility,  it  maintains  its  own  level 
and  keeps  itself  within  its  prescribed  bounds  ;  by  its 
buoyancy,  it  furnishes  a  habitation  for  numerous  tribes 
of  fishes,  and  lays  .the  foundation  of  the  whole  art  of 
navigation  ;  by  its  pressure  in  all  directions,  it  gives  the 
first  indication  of  containing  great  mechanical  energy, 
which  is  more  fully  developed  in  the  immense  force 
of  running  water,  which  may  be  regarded  as  a  reposi- 
tory of  power  kept  in  readiness  for  the  use  of  man  ; 
and,  finally,  by  its  property  of  being  converted  into 
steam,  it  discloses  a  new  and  inexhaustible  fountain  of 
mechanical  force,  which  man  may  employ  in  any 
degree  of  intensity  to  perform  the  humblest  and  the 
mightiest  of  his  works. 


CHAPTER   IV. 
PNEUMATICS. 

PROPERTIES    OF    ELASTIC    FLUIDS AIR-PUMP COMMON     PUMP SY- 
PHON  BAROMETER CONDENSER FIRE  ENGINE STEAM  AND  ITS 

PROPERTIES STEAM    ENGINE. 

85.  PNEUMATICS  is  that  branch  of  Natural  Philoso- 
phy which  treats  of  the  pressure  and  motion  of  elastic 
fluids.  Elastic  fluids  are  those  which  are  capable  of! 
contracting  or  dilating  their  volume  under  different 
degrees  of  pressure.  They  are  of  two  kinds,  gases 
and  vapors.  Gases  constantly  retain  the  elastic  in- 
visjble  state  •  vapors  remain  in  this  state  only  when 
heated  to  a  certain  degree,  but  return  to  the  liquid 

84.  Advantages  of  its  mobility — of  its  pressure — of  its  capacity  of 
being  converted  into  steam. 

;85.  Define  Pneumatics.  What  are  elastic  fluids  *?  State  the  two 
kinds  and  distinguish  between  them.  What  two  elastic  fluids  are 


PNEUMATICS.  85 

state  when  cooled.  Common  air  is  a  gas,  steam  a 
vapor.  Although  there  are  many  different  gases  and 
vapors  known  to  Chemistry,  yet  air  and  steam  are  the 
elastic  fluids  chiefly  regarded  in  Natural  Philosophy. 
Air  and  steam  are  both  commonly  invisible  ;  but  air, 
when  we  look  through  an  extensive  body  of  it,  ap- 
pears of  a  delicate  blue  or  azure  color,  which  habit 
leads  us  to  refer  to  distant  objects  seen  through  it.  It 
is  not  the  distant  mountain  that  is  blue,  but  the  air 
through  which  we  see  it.  Air  also  sometimes  becomes 
visible  when  ascending  and  descending  currents  mix, 
as  over  a  pan  of  coals,  or  a  hot  chimney,  when  we 
see  a  wavy  appearance,  which  is  air  itself.  Vapors 
also  exhibit  naturally  some  variety  of  colors,  as  yellow 
and  purple ;  but  the  vapor  of  water  or  steam  is  usually 
invisible.  We  must  carefully  distinguish  between 
elastic  vapor  and  the  mist  which  issues  from  a  tea- 
kettle. This  is  vapor  condensed,  or  restored  to  the  state 
of  water,  and  it  is  only  at  the  mouth  of  the  tea-kettle, 
where  it  is  hot,  that  it  is  in  the  state  of  steam,  and  there 
it  is  invisible. 

86.  The  general  principles  of  mechanics  apply  to 
liquids  and  gases,  as  well  as  to  solids,  all  bodies  being 
subject  alike  to  the  laws  of  motion  ;  but  the  property 
of  mobility  of  parts,  which  characterizes  liquids,  and 
of  elasticity  which  characterizes  gases  and  vapors, 
gives  them  severally  additional  properties,  which  lay 
the  foundation  of  hydrostatics  and  pneumatics.  Al- 
though we  do  not  usually  see  gases  and  vapors,  yet 
we  find  in  them  properties  of  matter  enough  to  prove 
their  materiality.  In  common  with  solids,  they  have 
impenetrability,  inertia,  and  weight ;  in  common  with 
liquids,  they  are  subject  to  the  law  of  equal  pressure 
in  all  directions,  and  when  confined  they  transmit  the 

chiefly  regarded  in  Natural  Philosophy  1  When  is  air  visible  1  Are 
vapors  ever  visible  1 

86.  Do  the  general  principles  of  Mechanics  apply  to  liquids  and 
ft 


86 


NATURAL   PHILOSOPHY. 


effects  of  a  pressure  or  blow  upon  any  one  part  of  th$ 
vessel,  to  all  parts  alike  ;  but  in  their  elasticity,  they 
differ  from  both  solids  and  liquids.  Since  air  and  steam 
are  the  elastic  fluids  with  which  Natural  Philosophy 
is  chiefly  concerned,  we  shall  consider  each  of  these 
separately. 

SEC.  1.  Of  Atmospheric  Air. 

87.  We  may  readily  verify  upon  atmospheric  air, 
the  various  properties  of  an  elastic  fluid.  Its  impene- 
trability, or  the  property  of  excluding  all  other  matter 
frorn  the  space  it  occupies,  will  be  manifested  if  we 
invert  a  tall  tumbler  in  water.  It  will  permit  the 
water  to  occupy  more  and  more  of  the  space  as  we 
depress  it  farther,  but  will  never  cease  to  exclude  the 
water  from  a  certain  portion  of  the  tumbler  which  it 
occupies.  We  may  render  this  ex- 
periment more  striking,  by  employ- 
ing a  glass  cylinder  and  piston,  as  is 
represented  in  Fig.  42.  Let  A  B  C  D 
represent  a  hollow  cylinder,  made 
perfectly  smooth  and  regular  on  the 
inside,  and  P  a  short  solid  cylinder, 
called  a  piston,  moving  up  and  down 
in  it  air-tight,  and  R  the  piston-rod. 
Now  when  we  insert  the  piston  near 
the  top  of  the  cylinder,  the  space 
below  it  is  filled  with  air.  On  de- 
pressing the  piston,  the  air,  on  ac- 
count of  its  elasticity,  gives  way,  and  we  at  first  feel 
but  little  resistance ;  but  as  we  thrust  it  down  nearer 
to  the  bottom,  the  resistance  increases,  and  finally  he- 
eases  1  What  property  characterizes  liquids,  and  what  solids  7 
W  hat  properties  of  matter  have  gases  and  vapors  1 

87.  Show  how  air  is  proved  to  be  material.    Explain  Figure  42. 
State  the  different  principles  which  this  apparatus  is  capable  of 


PNEUMATICS.  87 

omes  so  great  that  we  cannot  depress  it  any  farther 
'by  the  strength  of  the  hand.  If  we  apply  heavy 
weights,  we  may  force  it  nearer  and  nearer  to  the 
bottom  of  the  cylinder ;  but  no  power  will  bring  it  into 
contact  with  the  bottom.  This  experiment  may  be 
so  varied  as  to  prove  several  things.  First,  it  shows 
that  air  is  impenetrable  ;  secondly,  that  it  may  be 
indefinitely  compressed — all  the  air  of  a  large  room 
might  be  reduced  to  a  thimble-full,  and  on  removing 
the  pressure,  it  would  immediately  recover  its  original 
volume ;  thirdly,  that  the  resistance  increases  the  more 
it  is  compressed.  We  will  graduate  the  cylinder  into 
a  thousand  equal  divisions,  by  horizontal  marks  num- 
bered from  the  bottom  upward  from  one  to  one  thou- 
sand, and  place  on  the  pan  at  the  top  of  the  piston-rod 
a  few  grains,  so  as  just  to  overcome  the  friction  of 
the  piston  against  the  sides  of  the  cylinder.  We  will 
now  put  on  weights  successively,  until  we  have  sunk 
the  piston  half  way,  when  the  air  occupies  five  hun- 
dred instead  of  a  thousand  parts  of  the  cylinder.  If 
we  double  the  weight,  it  will  not  carry  the  piston  the 
same  distance  as  before,  that  is  to  the  bottom,  but  only 
through  half  the  remaining  space,  so  that  the  air  now 
occupies  one  fourth  of  the  capacity  of  the  cylinder. 
Jf  we  double  the  present  weight,  it  will  again  be  com- 
pressed one  half,  so  as  to  fill  but  an  eighth  part  of  the 
cylinder.  We  find,  therefore,  that  a  double  force 
of  compression,  always  reduces  to  half  the  former 
volume.  This  law  is  expressed  by  saying,  that  the 
volume  of  a  given  weight  of  air  is  inversely  as  the  com- 
pressing force. 

88.  Air  has  the  property  of  inertia.     It  remains  at 

proving.    How  is  the  volume  of  a  given  weight  of  air  proportioned 
to  the  compressing  Ibrce  1 

88.  -Why  has  air  the  property  of  inertia  *?  State  the  experiment 
which  shows  that  air  has  weight.  Why  is  air  called  a  fluid  1  Have 
the  particles  of  elastic  fluids  any  cohesion  1 


88  NATURAL   PHILOSOPHY. 

rest  unless  put  in  motion  by  some  force,  and  continues 
to  move  until  some  adequate  force  stops  it.*  When 
put  in  motion  by  any  moving  body,  it  destroys  just  as 
much  motion  in  that  body  as  it  receives  from  it ;  and 
it  loses  its  motion  only  as  it  imparts  the  same  amount 
to  some  other  matter.  A  large  body  moving  swiftly 
through  the  air  meets  with  great  resistance  ;  but  what- 
ever motion  it  loses,  it  imparts  to  the  air,  which  might 
be  sufficient  to  produce  a  high  wind.  Air  also  has 
weight.  If  we  balance  a  light  bottle,  containing  a  hun- 
dred cubic  inches,  in  a  delicate  pair  of  scales,  having 
just  pumped  out  all  the  air  from  the  bottle,  and  them 
open  the  stopper,  and  admit  the  air  again,  we  find  the 
vessel  has  gained  in  weight  30|-  grains.  We  call  air 
and  all  other  gases  and  vapors  fluids,  because  their 
particles  move  so  easily  among  themselves.  The  par- 
ticles of  elastic  fluids  have  no  cohesion,  but  on  the  other 
hand,  have  a  mutual  repulsion,  which  causes  them  to 
fly  off  from  each  other  as  soon  as  the  compressing  force 
is  removed  or  diminished. 

89.  The  lower  portions  of  air  which  lie  next  to  the 
earth,  are  pressed  by  the  whole  weight  of  the  atmo- 
sphere, which  is  found  to  amount  to  the  enormous  force 
of  15  pounds  upon  every  square  inch;  or  above  2,000 
pounds  upon  a  square  foot.    This  force  would  be  insup- 
portable to  man  and  animals,  were  it  not  equal  in  all 
directions,  entering  into  the  pores  of  bodies,  and  thus 
being  everywhere  nearly  in  a  state  of  equilibrium.     It 
is  only  when  we  withdraw  the  air  from  a  given  space, 
so  as  to  leave  the  surrounding  air  unbalanced,  that  we 
see  marks  of  this  violent  pressure. 

90.  THE  AIR-PUMP. — Various  properties  of  the  air 
are  exhibited  by  this  beautiful  and  interesting  appara- 
tus.    A  simple  form  of  the  Air-Pump  is  shown  in  fig- 


89.  What  is  the  pressure  of  the  atmosphere  upon  a  square  inch, 
and  square  foot  1    Why  is  it  not  insupportable  to  man  1 


PNEUMATICS. 


89 


tire  43.     A  represents   a   cylinder  having   a  piston 
moving  up  and  down  in  it.     The  cylinder  communi- 

Fig.  43. 


cates  by  an  open  pipe,  B,  with  the  plate  of  the  pump, 
C,  opening  into  the  receiver,  D,  which  is  a  glass  ves- 
sel ground  at  the  bottom  so  as  to  fit  the  plate  of  the 
pump  air-tight.  At  S  is  a  small  screw  which  opens  or 
closes  a  passage  into  the  pipe,  B,  by  which  air  may 
be  let  into  the  receiver  when  it  has  been  withdrawn  by 
the  pump. 

In  order  to  understand  how  the  pump  extracts  the 
air  from  the  receiver,  or  exhausts  it,  it  is  necessary 
first  to  learn  the  structure  of  a  valve.  A  valve  is  any 
contrivance  by  which  a  fluid  is  permitted  to  flow  one 
way,  but  prevented  from  flowing  the  opposite  way. 
A  common  hand  bellows  affords  an  example  of  a  valve, 
in  the  little  clapper  on  the  under  side.  When  the 
bellows  is  opened,  the  clapper  rises  and  the  air  runs 
in  ;  and  when  the  bellows  is  shut,  the  clapper  closes 

90.  Air-Pump.  Describe  Fig.  43.  Describe  a  valve.  Example  in 
a  bellows — in  the  piston  and  cylinder  of  the  air-pump.  Explain  the 


90  NATURAL  PHILOSOPHY. 

upon  the  orifice,  and  as  the  air  cannot  escape  by  the 
same  way  it  entered,  it  is  forced  out  by  the  nozzle  of 
the  bellows.  In  the  bottom  of  the  cylinder,  A,  in  fig- 
ure 43,  there  is  a  small  hole,  like  a  pin  hole.  On 
drawing  up  the  piston,  the  space  below  it  would  be  a 
vacuum  were  it  not  that  the  air  instantly  rushes  in 
from  the  pipe,  B,  and  the  receiver,  D,  and  fills  the 
space,  as  water  runs  into  a  syringe.  A  strip  of  oiled 
silk  is  tied  firmly  over  the  orifice  in  the  bottom  of 
the  cylinder  on  the  inside,  opening  freely  upward 
when  this  air  seeks  entrance  from  below,  but  shutting 
downward  and  preventing  its  return.  Then  if  we 
should  attempt  to  force  down  the  cylinder,  the  air 
below  it  would  resist  its  descent ;  but  a  small  hole  is 
made  through  the  piston  itself,  and  a  valve  tied  to  the 
upper  side  opening  upward ;  so  that  on  depressing 
the  piston,  the  air  below  makes  its  way  through  the 
valve  and  escapes  into  the  open  space  above.  We 
raise  the  piston,  and  the  air  in  the  receiver  follows  it 
through  a  valve  in  the  bottom  of  the  cylinder  opening 
upward.  The  original  air  of  the  receiver  being  now 
expanded  equally  through  the  receiver,  the  cylinder, 
and  the  connecting-pipe,  we  thrust  down  the  piston, 
and  the  portion  of  the  air  that  is  contained  in  the  cyl- 
inder is  forced  out  through  the  piston.  We  again 
raise  the  piston,  and  the  remaining  air  of  the  receiver 
expands  itself  as  before  through  the  vacuum ;  we 
depress  the  piston,  and  a  second  cylinder  full  of  air 
is  withdrawn.  By  continuing  this  process,  we  rarefy 
more  and  more  the  air  of  the  receiver,  every  stroke  of 
the  piston  leaving  what  remains  more  rare  than  be- 
fore. Still,  on  account  of  the  elasticity  of  air,  what 
remains  in  the  cylinder  will  always  diffuse  itself 
through  the  whole  vessel,  so  that  we  cannot  produce 
a  complete  vacuum  by  the  air-pump. 

process  of  exhausting  a  vessel.  Can  we  produce  a  complete  va- 
cuum by  the  air-pump !? 


PNEUMATICS.  91 

91.  Several  experiments  will  illustrate  the  great 
pressure  of  the  atmosphere,  when  no  longer  balanced 
by  an  equal  and  opposite  force.  We  shall  find  the 
receiver,  when  exhausted  by  the  foregoing  process, 
held  firmly  to  the  plate  of  the  pump  so  that  we  cannot 
remove  it  until  we  have  opened  the  screw,  S,  and 
admitted  the  air;  then  the  downward  force  of  the  air 
being  counterbalanced  by  an  equal  force  from  within, 
the  vessel  is  easily  taken  off.  The 
Magdeburg  Hemispheres,  represented  in  Fig.  44. 
figure  44,  afford  a  striking  illustration 
of  the  force  of  atmospheric  pressure. 
When  they  have  air  within  as  well  as  , 
without,  they  are  easily,  when  joined, , 
•separated  from  each  other ;  but  let  us  ' 
now  put  them  closely  together  and 
screw  the  ball  thus  formed  upon  the 
plate  of  the  pump,  exhaust  the  air,  and 
^close  the  stop-cock  so  as  to  prevent  its 
return.  We  then  unscrew  the  ball  from  the  pump,  and 
.screw  on  the  loose  handle  ;  the  hemispheres  are  pressed 
so  closely  together  that  two  men,  taking  hold  by  the 
opposite  handles,  can  hardly  pull  them  apart.  Hemis- 
pheres four  inches  in  diameter  would  be  held  togeth- 
er with  a  force  equal  to  188  pounds.  Otto  Guericke, 
of  Magdeburg,  in  Germany,  who  invented  the  air-pump 
and  contrived  this  experiment,  had  a  pair  of  hemis- 
pheres constructed,  so  large  that  sixteen  horses,  eight 
on  each  side,  were  unable  to  dratv  them  apart.  A 
pair  only  two  feet  in  diameter,  would  require  to  sepa- 
rate them  a  force  equal  to  6785  pounds.  If  our  bod- 
ies were  not  so  penetrated  by  air,  that  the  external 
pressure  is  counterbalanced  by  an  equal  force  from 

91.  Give  an  example  of  the  great  pressure  of  the  atmosphere.  De- 
scribe the  Magdeburg  Hemispheres.  What  is  said  of  tt^se  made 
by  Otto  Guericke  1  How  much  pressure  does  a  middle-sized  man 
sustain  1  Why  are  we  not  crushed  1 


92  NATURAL  PHILOSOPHY. 

within,  we  should  be  crushed  under  the  weight  of 
the  atmosphere ;  for  a  middle  sized  man  would  sus- 
tain a  pressure  of  about  14  tons. 

92.  If  we  take  a  square  bottle,  fit  a  stop- cock  to  it, 
and  exhaust  the  air,  the  pressure  on  the  outside  will 
crush  it  into  small  fragments,  with  a  loud  explosion. 
It  is  prudent  to  throw  a  towel  or  handkerchief  loose- 
ly over  it,  to  prevent  injury  from  the  fragments.  A 
square  bottle  is  preferred  to  a  round  one,  because 
such  a  figure  has  less  power  of  resistance.  The  lap- 
stone  experiment  may  be  tried  without  an  air-pump, 
and  affords  a  pleasing  illustration  of  the  force  of 
atmospheric  pressure.  Cut  out  a  circular  piece  of 
sole  leather,  five  or  six  inches  in  diameter.  Through 
a  hole  in  the  center  draw  a  waxed  thread  to  serve  as 
a  handle.  Soak  the  leather  in  water  until  it  is 
very  soft  and  pliable ;  then,  on  applying  this  to  any 
smooth,  clean  surface,  as  that  of  a  lap-stone,  a  slab 
of  marble,  or  a  table,  it  will  adhere  with  such  force, 
that  we  cannot  lift  it  off;  but  when  we  pull  up- 
ward, the  heavy  body  to  which  it  is  attached  will 
be  lifted  with  it.  We  may,  however,  slide  it  with 
ease,  because  no  force  acts  up- 
Fig.  45.  on  it  to  prevent  its  motion  in 

this  direction,  except  simply 
the  adhesion  of  the  surfaces. 
Flies  are  said  to  ascend  a  pane 
of  glass  on  this  principle,  by 
applying  their  broad  feet  firmly 
to  the  glass,  which  are  held 
down  by  the  pressure  of  the  at- 
mosphere. When  we  apply  a 
'  sucker,  and  exhaust  it  with  the 
mouth,  the  fluid  rises  because 

92.  Describe  the  experiment  with  a  square  bottle.  Also  the  lap- 
etone  experiment.  Why  can  we  so  easily  slide  the  leather  1  How 
do  flies  ascend  smooth  planes  1  How  does  the  boy  suck  water  1 


PNEUMATICS. 


93 


Fig.  46. 


it  is  forced  up  by  the  pressure  of  the  atmosphere  on  its 
surface.  When  we  draw  in  the  breath,  the  lungs  are 
expanded  like  a  pair  of  bellows.  Thus  the  air  runs 
from  the  sucker  into  the  lungs,  and  forms  a  vacuum  in 
the  sucker.  Immediately  the  pressure  of  the  atmo- 
sphere on  the  surface  of  the  fluid,  not  being  balanced 
in  the  tube,  forces  the  fluid  up  the  tube  and  thence  into 
the  mouth. 

93.  If  we  fill  a  vial  with  water,  and,  placing  one 
thumb  on  the  mouth,  invert  it  in  a  tumbler  partly  full 
of  water,  the  water  will  not  run  out  of 
the  vial,  but  will  remain  suspended, 
because  there  being  no  air  at  the  top  of 
the  column  to  balance  the  pressure  that 
acts  at  the  mouth  of  the  vial,  the 
column  cannot  descend.  If,  however, 
instead  of  the  vial,  we  should  employ  a 
pipe  more  than  33  feet  long,  on  filling  it 
and  inverting  it,  as  was  done  with  the 
Vial,  the  water  would  settle  to  about  33 
feet,  and  there  it  would  rest ;  for  the 
pressure  of  the  atmosphere  is  capable  of 
sustaining  a  column  of  water  only  33 
feet  high.  Were  it  higher  than  this,  it 
would  be  more  than  a  counterpoise  for 
that  pressure,  and  would  overcome  it 
and  sink  ;  and  were  it  lower  than  that, 
it  would  be  overcome  by  that  pressure, 
and  rise  until  it  exactly  balanced  the 
force  of  the  atmosphere.  Instead  of  fill- 
ing the  pipe  with  water,  we  will  attach  a 
stop-cock  to  the  open  end,  screw  it  on 
i  the  plate  of  the  air-pump,  and  exhaust  the  air.  We 
I  will  now  close  the  stop-cock,  and  removing  the  tube 

93:  Describe  the  experiment  with  the  vial.    Also  with  a  pipe 
more  than  thirty-three  feet  long.    If  we  exhaust  the  pipe  arid  open 
|    it  under  water,  what  happens  1 


NATURAL  PHILOSOPHY. 


from  the  pump,  will  place  the  lower  end  of  the  pipe 
in  a  bowl  of  water.  On  opening  the  stop-cock  the 
water  will  rush  into  the  pipe,  and  rise  to  about  the 
same  height  as  before,  namely,  about  33  feet,  where 
it  will  rest.  In  both  cases,  there  is  an  empty  space  or 
vacuum  in  the  upper  part  of  the 'pipe  above  the  column 
of  water. 

Fig.  47.          94.  This  experiment  illustrates  the  prin- 
ciple of  the   common  pump,  of  the  syphon, 
and  of   the    barometer.     Let  us  first  <:see 
how  water  is   raised   by  the  pump.     This 
"S    apparatus  usually  consists  of  two  pipes — 
^=A  a  larger,  A  B,  above.,  and  a  smaller,  H  C, 
~°  below.     The  piston  moves  in  the  larger 
.     pipe,  and  the  smaller  pipe   descends  into 
the  well.     On  the  top  of  the  latter,  where 
it  enters  the  former,  is  a  valve,  V,  opening 
upward.     Suppose  the  piston,   P,  is  down 
p     close  to  this  valve.     On  raising  it,  the  air 
from  the  lower  pipe  diffuses  itself  into  the 
r     empty  space  below  the   piston,    becomes 
H     rarefied,  and  no  longer  balances  the  pres- 
sure of  the  atmosphere  on  the  surface  of 
B     the    well.     Consequently,    the    water    is 
forced  up  until  the  weight  of  the  column, 
together  with  the  weight  of  the    rarefied 
air,  restores  the  equilibrium.     Suppose  by 
the  piston  being  drawn  up  to  P,  the  water 
rises  to  H  ;  then  the  column,  H  C,  and  the 
rarefied    air  in    both  pipes  together,  first 
counterbalance  the  weight  of  the    atmo- 
sphere.    On  raising  the  piston  still  higher, 
the  water  rises  above  H,  but  would  not  prob- 
ably reach  the  valve,  V,  by  a  single  elevation  of  the  pis- 


H 


94.  Explain  the  common  pump  from  Fig.  47.    How  much  force 
does  the  atmosphere  exert  in  raising  the  water  1 


PNEUMATICS. 


ton.  We  therefore  thrust  down  the  piston  to  repeat  the 
operation.  The  air  between  V  and  P  is  prevented  from 
returning  into  the  lower  pipe,  by  the  valve,  V,  which 
shuts  downward  ;  but  the  enclosed  air,  when  com- 
pressed by  the  descending  piston,  lifts  a  valve  in  the 
piston,  as  in  the  air-pump,  and  escapes  above.  On- 
drawing  up  the  piston  a  second  time,  suppose  that  the 
water  rises  into  the  upper  pipe  above  the  valve,  V,  then 
on  depressing  the  piston  again,  this  water,  pressed  on 
by  the  piston,  lifts  its  valve,  and  gets  above  it.  Finally, 
on  drawing  up  the  piston  again,  this  same  water  is  lifted 
up  to  the  level  of  the  spout,  S,  where  it  runs  off.  We 
exert  just  as  much  force  in  exhausting  the  air,  as  the 
pressure  of  the  atmosphere  exerts  in  raising  the  wa- 
ter. It  requires,  therefore,  just  as  much  force  to  raise 
a  given  quantity  of  water  by  the  pump,  as  to  draw  it 
up  in  a  bucket ;  and  the  only  question  is,  which  is  the 
most  convenient  mode  of  applying  the  force. 

95.  The  Syphon  is  a  bent  tube, 
having  one  leg  longer  than  the 
other,  as  in  Fig.  48.  If  we  dip 
[the  shorter  leg  into  water  and 
Isuck  out  the  air  from  the  tube, 
nhe  water  will  rise,  pass  over  the 
t;bend,  flow  out  at  the  open  end, 
;and  continue  to  run  until  all  the 
] water  in  the  vessel  is  drawn  off. 
IHere  the  pressure  of  the  atmo- 
jsphere  on  both  mouths  of  the  tube 
iis  the  same  ;  but  in  each  arm,  that 
jpressure  is  resisted  by  the  weight 
jof  the  column  of  water  above  it,  and  more  by  the 
jlonger  than  by  the  shorter  column.  This  is  the  same 
Uhing  as  though  the  pressure  were  less  upon  the  outer 


P5.  Describe  the  Syphon.  Why  does  it  draw  off  the  liquid'?  State 
he  uses  of  the  Syphon.     How  high  will  it  raise  water  1 


96  NATURAL   PHILOSOPHY. 

than  upon  the  inner  mouth ;  and  it  is  easy  to  see  that! 
if  the  water  in  a  tube  is  pressed  one  way  more  thad 
the  other,  it  will  flow  in  the  direction  in  which  the- 
pressure  is  greatest.  The  syphon  is  used  in  dra wing- 
off  liquors ;  and  the  water  in  aqueducts  is  sometimes; 
conveyed  over  hills  on  the  principle  of  the  syphon., 
F.  4q  But  we  must  remember,  that  water  couldj 
riot  be  raised  by  it  more  than  33  feet ;  for] 
when  the  bend  is  33  feet  above  the  level  of 
-si  the  fountain,  then  the  column  in  the  shorter 
arm  balances  the  pressure  of  the  atmosphere 
30  at  the  mouth  of  the  tube  in  the  well,  and 
29  leaves  no  force  to  drive  forward  the  column* 

into  the  descending  arm. 

28  96.  The  Barometer  is  an  instrument  for 
27  measuring  the  pressure  of  the  atmosphere.  If 
the  atmosphere  be  conceived  to  be  divided^ 
into  perpendicular  columns,  the  barometer* 
measures  the  weight  of  one  of  these  by  the* 
height  of  a  column  of  quicksilver  which  it 
takes  to  balance  it.  Quicksilver  is  13-i-  times| 
as  heavy  as  water,  and  therefore  a  column  so* 
much  shorter  than  one  of  water,  will  balance] 
the  weight  of  an  atmospheric  column.  This 
will  imply  a  column  about  2i  feet,  or  3flj 
inches  high ;  and  it  will  be  much  more  conJ 
venient  to  experiment  upon  such  a  columnj 
than  upon  one  of  water  33  feet  high.  W6| 
will  therefore  take  a  glass  tube  about  three* 
feet  long,  closed  at  one  end  and  open  at  the* 
other,  fill  it  with  quicksilver,  and  placing  the 
finger  firmly  on  the  open  mouth,  we  will  in- 
sert this  below  the  surface  of  the  fluid  in  the 
small  cistern,  as  represented  in  the  figure. 

96.  Define  the  Barometer.  Describe  the  mode  of  making  it  by 
Fig.  49.  At  what  height  will  the  quicksilver  rest  1  What  is  the 
space  above  it  called  1 


PNEUMATICS.  97 

On  withdrawing  the  finger,  the  quicksilver  in  the  tube 
will  settle  to  the  height  of  about  thirty  inches,  where 
it  will  rest,  being  sustained  by  the  pressure  of  the 
atmosphere  on  the  surface  of  the  fluid  in  the  cistern, 
to  which  force  its  weight  is  exactly  equal.  The  space 
above  the  quicksilver,  is  the  best  vacuum  we  are  able 
to  form.  It  is  called  the  Torricellian  vacuum,  from 
Torricelli,  an  Italian  philosopher,  who  first  formed  it. 
The  weight  of  a  column  of  atmospheric  air  is  different 
in  different  states  of  weather,  and  its  variations  will  be 
indicated  by  the  rising  and  falling  of  the  quicksilver  in 
the  barometer.  Any  increase  of  weight  in  the  air 
will  make  the  fluid  rise  ;  any  diminution  of  weight 
will  make  it  fall.  Hence,  these  variations  in  the 
height  of  the  barometric  column,  show  us  the  compara- 
tive weight  and  pressure  of  the  atmosphere  at  any 
given  time.  By  applying  to  the  upper  part  of  the  tube 
a  scale  divided  into  inches  and  tenths  of  an  inch,  we 
can  read  off  the  exact  height  of  the  quicksilver  at  any 
given  time.  Thus,  the  fluid,  as  represented  in  the 
figure,  stands  at  29.4  inches. 

97.  The  barometer  is  one  of  the  most  useful  and 
instructive  of  philosophical  instruments.  By  observing 
it  from  time  to  time,  we  may  find  how  its  changes  are 
connected  with  the  changes  of  weather,  and  thus  it 
frequently  enables  us  to  foretell  such  changes.  If,  for 
example,  we  should  observe  a  sudden  and  extraordinary 
fall  of  the  barometer,  we  should  know  that  a  high 
wind  was  near,  possibly  a  violent  gale.  To  seafaring 
men,  the  barometer  is  a  most  valuable  instrument, 
since  it  enables  them  to  foresee  the  approach  of  a  gale, 
and  provide  against  it.  As  a  general  fact,  the  rising 
of  the  barometer  indicates  fair,  and  its  falling,  foul 
weather. 

97.  Explain  the  use  of  the  barometer  as  a  weather  glass.    "What 
would  a  sudden  and  extraordinary  fall  indicate  1     What  weather 
does  its  rise,  and  what  its  fall  indicate  1 
9 


98 


NATURAL  PHILOSOPHY. 


98.   The    foregoing    considerations    relate    to  the 
weight  and  pressure  of  the  atmosphere  ;  but  the  air- 
pump  also  affords  us  interesting  illustrations  of  the 
elasticity  of  air.     We  will  fill  a 
Fig.  50.  vial  with  water,  and  invert  it  in 

a   tumbler   partly  filled  with   the 
C~)  same  fluid.     We  will  now  place 

/*v    >k  the  tumbler  and  vial  on  the  plate 

of  the  air-pump,  and  cover  it  with 
a  receiver,  and  exhaust  the  air. 
Soon  after  we  begin  to  work  the 
pump,  we  shall  see  minute  bubbles 
of  air  making  their  appearance  in 
the  water,  which  will  rise  and  col- 
^^_r~u--  -«.~—  ^ect  *n  a  bubble  at  the  top  of  the 
column.  The  bubble  thus  formed,  will  expand  more 
and  more  as  the  exhaustion  proceeds,  until  it  expels 
the  water,  and  occupies  the  whole  interior  of  the  viaL 
This  will  happen  much  sooner  if  we  let  in  a  bubble  of 
air  at  first,  and  do  not  wait  for  it  to  be  extricated  from 
the  water ;  but  this  extrication  of  air  from  the  water, 
is  itself  an  instructive  part  of  the  experiment,  as  it 
shows  us  that  water  contains  a  large  quantity  of  air, 
held  in  combination  with  it  by  the  pressure  of  the 
atmosphere  on  the  surface,  which  pressure  pervades 
all  parts  of  the  fluid  alike.  But  on  withdrawing  this 
pressure  gradually  from  the  surface  of  the  water,  the 
particles  of  air  imprisoned  in  the  pores  of  the  water 
escape,  and  collect  on  the  top.  The  bubble  thus 
formed,  will  expand  more  and  more  as  the  pressure  is 
still  farther  removed,  until  it  drives  down  the  water 
and  fills  the  whole  vial.  If  we  turn  the  screw  S  of 
the  pump  (Fig.  43)  and  let  in  the  air,  the  pressure  on 
the  surface  of  the  water  in  the  tumbler  being  restored, 

98.  Describe  Fig.  50,  and  shovy  how  it  illustrates  the  elasticity  of 
air.  What  will  porous  bodies  give  out  in  an  exhausted  receiver  1 
How  will  warm  water  be  affected  1 


PNEUMATICS.  99 

the  water  will  be  forced  up  the  vial  again,  and  the 
air  will  be  reduced  to  its  original  bubble.  If  we  place 
any  porous  substance,  as  a  piece  of  brick,  or  a  crust 
of  bread,  in  a  tumbler,  and  fill  the  tumbler  with  water, 
(attaching  a  small  weight  to  the  bread  to  keep  it  under) 
we  shall  see,  in  like  manner,  an  unexpected  amount  of 
air  extricated  when  we  place  it  under  the  receiver,  and 
remove  the  atmospheric  pressure  from  it,  so  as  to  per- 
mit it  to  assume  the  elastic  state.  Liquids  boil  at  a 
much  lower  temperature  than  usual,  when  the  pressure 
of  the  atmosphere  is  removed  from  them.  Thus,  if 
we  take  a  tumbler  half  full  of  water,  no  more  than 
blood- warm,  set  it  under  the  receiver,  and  exhaust  the 
air,  it  will  boil  violently. 

99.  Air  is  the  medium  of  combustion,  of  respiration) 
and  of  sound.     If  we  place  a  lighted  candle  under  the 
receiver  of  an  air-pump,  and  exhaust  the  air,  the  light 
will  immediately  go  out,  showing  that  bodies  cannot 
burn  without  the  presence  of  air.     Nor  without  this 
can  animals  breathe.     A  small  bird  placed  beneath 
the  receiver,  will  cease  to  breathe  as  soon  as  the  air  is 
exhausted.     If  a  bell,  also,  is  made-,  to  ring  under  a 
receiver,  the  sound  will  grow  fainter  and  fainter  as 
the  air  is  withdrawn,  and  finally  be  scarcely  heard  at 
all.     The  buoyancy  of  air,  like  that  of  water,  enables 
it  to  support  light  bodies.     In  a  vacuum,  the  heaviest 
and  lightest  bodies  descend  to  the  earth  with  the  same 
velocity.      If  we  suspend  a  guinea  and  a  feather  from 
the  top  of  a  tall  receiver,  exhaust  the  air,  and  let  them 
fall  at  the  same  instant,  the  feather  will  keep  pace  with 
the  guinea,  and  reach  the  plate  of  the  pump  at  the 
same  instant. 

100.  THE  CONDENSER. — A  piston  and  cylinder  may 
be  so  contrived  as  to  pump  air  into  a  vessel  instead  of 

99.  How  may  we  show  that  air  is  essential  to  combustion'?    Also 
to  life'?  Also  to  sound  1  Describe  the  guinea  and  feather  experiment 


100 


NATURAL  PHILOSOPHY. 


Fig.  51. 


pumping  it  out.  Figure  51  represents  a  condensing 
syringe,  screwed  to  a  box  partly  filled  with  water. 
When  the  piston  is  drawn  up  to  the  top,  above  an  ori- 
fice E  in  the  side,  the  air  runs  in  at 
E,  which  on  depressing  the  piston,  is 
driven  forward  into  the  box  through 
a  valve,  V,  which  opens  inward,  but 
closes  outward,  and  prevents  the  re- 
turn of  the  air.  By  repeated  blows 
of  the  piston,  more  and  more  air  is 
forced  into  the  box,  constantly  in- 
creasing the  pressure  on  the  surface 
of  the  water.  D  is  a  tube  opening 
and  closing  by  a  stop-cock,  having  its 
lower  end  in  the  water.  When  the 
air  is  strongly  condensed,  on  opening 
the  stop-cock,  the  water  issues  from 
the  tube  with  violence.  Soda  Water 
Fountains  are  constructed  on  this  prin- 
ciple. A  great  quantity  of  carbonic 
acid,  or  fixed  air,  is  forced  into  a 
strong  metallic  vessel,  containing  a 
solution  of  soda,  and  therefore  is  sub- 
jected to  a  powerful  pressure.  A  tube  connects  this 
vessel  to  the  counter  where  the  liquor  is  to  be  drawn, 
which  issues  with  violence,  as  soon  as  vent  is  given  to 
it,  and  foams,  in  consequence  of  the  carbonic  acid 
expanding  by  the  removal  of  the  pressure  by  which  it 
had  been  confined.  The  condenser  employed  for  this 
purpose,  is  called  a  forcing  pump,  and  differs  from  the 
condensing  syringe,  represented  in  figure  51,  chiefly  in 
being  worked  by  a  lever  attached  to  the  piston,  instead 
of  the  naked  hand. 


100.  Describe  the  Condenser  from  Fig.  51.    How  is  air  pumped 
into  the  box  *?    Explain  the  principle  of  soda  water  fountains. 

What  is  the  forcing  pump  1 


PNEUMATICS. 

-Fig.  52. 


101 


101.  The  Fire-Engine  throws  water  by  means  of 
two  forcing  pumps,  one  on  each  side,  which  are  work- 
ed  by  the  firemen.  T  represents  the  hose,  or  leath- 
ern pipe,  which  leads  off  to  some  well  or  cistern  of 
water,  whence  the  supply  is  drawn.  F  is  the  work- 
ing beam,  to  each  end  of  which  is  attached  a  piston 
moving  in  the  cylinder  A  B.  Suppose  at  the  com- 
mencement of  the  process,  the  left  hand  piston  is 
down  close  to  the  valve  V ;  as  it  rises,  the  water  fol- 
lows it  from  the  hose,  lifting  the  valve  V,  and  enter- 
ing P  B  below  the  piston.  When  the  piston  descends, 
it  forces  the  water  through  a  valve  into  the  air- 
vessel,  M.  As  the  water  is  thrown  in  by  successive 
descents  of  the  piston,  it  rises  in  M,  and  condenses 
the  air  of  the  vessel  into  a  small  space  at  the  top.  A 
second  hose,  F,  dips  into  the  water,  and  terminates  in 
the  farther  end  in  a  pipe,  which  the  fireman  directs 


101.  Describe  the  fire-engine  from  Fig.  52. 
used  1    Use  of  air-springs  and  air-beds  1 
9* 


Why  is  the  air-vessel 


102  NATURAL  PHILOSOPHY. 

upon  any  required  point,  sending  the  water  in  a  con- 
tinual stream.  The  stream  might  indeed  be  propel- 
led directly  by  the  action  of  the  pistons,  without  the 
intervention  uf  the  compressed  air  in  M ;  but  in  that 
case  it  would  go  by  jerks ;  wHereas,  the  elasticity  of 
the  confined  air  acts  as  a  uniform  force,  and  makes 
the  water  flow  out  in  a  continual  stream.  Air-springs, 
acting  on  the  same  principle,  are  sometimes  attached 
to  coaches,  and  are  said  to  operate  well.  Beds  have 
been  filled  by  inflating  them  with  air  instead  of  feath- 
ers, and  have  the  advantage  of  being  always  made  up. 

SEC.  2.  Of  Steam  and  its  Properties. 

102.  Steam,  or  the  elastic  fluid  which  is  produced 
by  heating  water,  owes  its  mechanical  efficacy  to  its 
power  of  suddenly  acquiring  by  heat  a  powerful  elas- 
ticity, and  then  losing  it  as  suddenly,  by  cold ;  in 
the  former  case,  expanding  rapidly,  and  expelling 
every  thing  else  from  the  space  it  occupies ;  and,  in 
the  latter  case,  shrinking  instantly  to  its  original  di- 
mensions in  the  state  of  water,  and  thus  forming  a  va- 
cuum. By  this  means,  an  alternate  motion  is  given 
to  a  piston,  which  being  communicated  to  machinery, 
supplies  a  force  capable  of  performing  every  sort  of 
labor,  and  being  easily  endued  with  any  required  de- 
gree of  energy,  is  at  once  the  most  efficient  and  the 
most  manageable  of  all  the  forces  of  nature.  Thus, 
if  steam  be  admitted  below  the  piston,  in  figure  53, 
when  its  force  accumulates  sufficiently  to  overcome 
the  resistance  of  the  piston,  it  raises  it ;  and  if  it  then 
be  let  in  above  the  piston,  it  depresses  it.  When  the 
piston  rises,  it  may  be  made  to  turn  a  crank  half 
round,  and  the  other  half  when  it  falls,  and  thus  a 

102.  To  what  two  properties  does  steam  owe  its  mechanical  effi- 
cacy 1  To  what  is  trie  motion  first  communicated,  and  how  trans- 
ferred to  machinery  1  Show  how  the  piston  is  raised  and  depressed. 


PNEUMATICS.  103 

main  wheel  may  be  made  to  revolve,  from  which  mo- 
tion may  be  conveyed  to  all  sorts  of  machinery.  The 
degree  of  force  which  steam  exerts,  depends  on  the 
temperature  and  density  conjointly.  If  we  put  a 
spoonful  of  water  into  a  convenient  vessel,  as  an  oil- 
flask,  and  place  it  over  the  fire,  the  water  will  soon 
be  turned  into  elastic  vapor,  which  will  drive  out  the- 
air  and  fill  the  entire  capacity  of  the  vessel.  As  soon 
as  this  takes  place,  we  cork  the  flask  and  again  set  it 
over  the  fire.  The  steam  will  increase  in  elastic 
power,  just  as  that  of  air  would  do,  which  is  only  at 
a  moderate  rate,  and  it  might  be  heated  red  hot  with- 
out exerting  any  violent  force.  If  we  now  unstop  the 
flask  and  fill  it  one  third  full  of  water,  and  again  place 
it  on  the  fire,  and  stop  it  close  when  it  is  boiling  free- 
ly, then  successive  portions  of  water  will  be  constant- 
ly passing  into  vapor,  and,  of  course,  the  steam  in 
the  upper  part  of  the  vessel  will  be  constantly  growing 
more  and  more  dense.  It  is  important  to  remember, 
therefore,  that  when  steam  is  heated  by  itself,  and 
not  in  contact  with  water,  its  elasticity  increases 
slowly,  and  never  becomes  very  great;  but  when  it 
is  heated  in  a  close  vessel  containing  water,  which 
makes  to  it  constant  additions  of  vapor,  thus  increas- 
ing its  density,  it  rapidly  acquires  elastic  force,  and 
the  faster  the  longer  the  heat  is  continued,  so  as 
shortly  to  reach  an  energy  which  nothing  can  resist. 
!  Such  an  accumulation  of  force  sometimes  takes  place 
by  accident  in  a  steam  boiler,  and  produces,  as  is 
well  known,  terrible  explosions. 

103.  If  the  foregoing  principles    are   well    under- 
i  stood,  it  will  be  easy  to    learn  the  construction  and 
l  operation  of  the  Steam  Engine.     For  the  sake  of  sim- 
plicity, we  will  leave  out  numerous  appendages  which 


Upon  what  does  the  degree  of  force  depend  1    Experiment  with  a 
flask  of  steam  with  and  without  water. 


104 


NATURAL  PHILOSOPHY. 


usually  accompany  this  apparatus,  but  are  not  essen- 
tial to  the  main  principle.     In  figure  53,  A  represents 

Fig.  53. 


the  boiler,  C  the  cylinder,  in  which  the  piston  H  moves, 
L  the  condenser,  and  M  the  air-pump.     B  is  the  steam- 
pipe,   branching   into    two   arms,    communicating  re- 
spectively with  the  top  and  bottom  of  the  cylinder,  and;! 
K  is  the   eduction-pipe,  formed  of  the    two  branches  \ 
which  proceed  from  the  top  and  bottom  of  the  cylin- 
der on  the  other  side,  and  communicate  between  the 
pylinder  and  the  condenser,  which  is  immersed  in  a 
well  or  cistern  of  cold  water.     Each   branch  of  the 
pipe  has  its  own  valve,  as  F,  G,  P,  Q,  which  may  be  ' 
opened  or  closed  as  occasion  requires.     R  is  a  safety 
valve,  closed  by  a  plate,   which  is  held  down  by  a 
weight  attached  to  a  lever,    and  sliding  on  it,  so  as 
to  increase  or  diminish  the  force  at  pleasure.     When 


103.  Describe  Figure  53. 


PNEUMATICS.  105 

the  force  of  the  steam  exceeds  this,  it  will  lift  the 
valve  and  escape,  thus  preventing  the  danger  of  explo- 
[sion. 

104.  Suppose,  first,  that  all  the  valves  are  open,  and 
that  steam  is  issuing  freely  from  the  boiler.  It  is  easy 
to  see,  that  the  steam  would  circulate  freely  through 
all  parts  of  the  engine,  expelling  the  air,  which  would 
escape  through  the  valve  in  the  piston  of  the  air-pump, 
and  thus  the  interior  spaces  would  be  all  filled  with 
j steam.  This  process  is  called  blowing  off ;  it  is  heard 
)  when  a  steamboat  is  about  leaving  the  wharf.  Next 
*.the  valves,  F  and  Q,  are  closed,  G  and  P  remaining 
open.  The  steam  now  pressing  on  the  cylinder,  forces 
fit  down,  and  the  instant  when  it  begins  to  descend,  the 
istop-cock  O  is  opened,  through  which  cold  water  meets 
the  steam  as  it  rushes  from  the  cylinder  and  condenses 
it,  leaving  no  force  below  the  piston  to  oppose  its  descent. 
Lastly,  G  and  P  being  closed,  F  and  Q  are  opened,  the 
steam  flows  in  from  the  boiler  below  the  piston,  and 
(rushes  from  above  into  the  condenser,  by  which  means 
jthe  piston  is  forced  up  again  with  the  same  power  as 
Jthat  by  which  it  descended.  Meanwhile,  the  air-pump 
jis  playing,  and  removing  the  water  and  air  from  the 
'condenser,  and  pouring  the  water  into  a  reservoir, 
'whence  it  is  conveyed  to  the  boiler  to  renew  the  same 

rircuit. 
105.  In  High  Pressure  engines,  the  steam  is  not 
Icondensed,  but  discharges  itself  directly  into  the  atmo- 
sphere. The  puffing  heard  in  locomotives,  arises  from 
•this  cause.  High  pressure  engines  are  those  in  which 
isteam  of  great  density,  and  high  elastic  power,  is  used. 
,By  this  means,  a  more  concentrated  force  is  produced, 
jand  the  engine  may  be  smaller  and  more  compact ; 

104.  Show  how  the  engine  is  set  a  going,  and  kept  at  work. 
;    105.  What  becomes  of  the  steam  in  high  pressure  engines  1  Whence 
arises  the  puffing  heard  in  locomotives  *    What  are  high  pressure 
engines  1    What  are  their  advantages  over  low  pressure  engines  1 


106  NATURAL  PHILOSOPHY. 

but  unless  it  is  made  proportionally  stronger,  it  is  more 
liable  to  explode,  and  when  it  gives  way  it  explodes 
with  great  violence. 


CHAPTER  V. 
METEOROLOGY. 

GENERAL    OBJECTS  OF  THE    SCIENCE — EXTENT,   DENSITY,  AND    TEM- 
PERATURE   OF    THE    ATMOSPHERE ITS    RELATIONS    TO    WATER 

RELATIONS   TO    HEAT RELATIONS   TO    FIERY    METEORS. 

106.  METEOROLOGY  is  that  branch  of  Natural  Phi- 
losophy which  treats  of  the  Atmosphere.  In  Pneu- 
matics, we  learn  the  properties  of  elastic  fluids  in 
general,  on  a  small  scale,  and  by  experiment  rather 
than  by  observation  ;  but  in  Meteorology,  we  extend 
our  views  to  one  of  the  great  departments  of  nature, 
and  we  reason,  from  the  known  properties  of  air  and  , 
vapor,  upon  the  phenomena  and  laws  of  the  entire j 
body  of  the  air,  or  the  atmosphere.  Meteorology  leads! 
us  to  consider,  first,  the  description  of  the  atmosphere  1 
itself,  including  its  extent,  condition  at  different  heights, 
and  the  several  elements  that  compose  it ;  secondly, 
the  relations  of  the  atmosphere  to  water,  including 
the  manner  in  which  vapor  is  raised  into  the  atmo- 
sphere, the  mode  in  which  it  exists  there,  and  the 
various  ways  in  which  it  is  precipitated  in  the  form 
of  dew,  fog,  clouds,  rain,  snow,  and  hail ;  thirdly,  the 
relations  of  the  atmosphere  to  heat,  embracing  the 
motions  of  the  atmosphere  as  exhibited  on  a  small 
scale,  in  artificial  draughts  and  ventilation  ;  and  on 
a  large  scale,  in  winds,  hurricanes,  and  tornadoes  ; 


106.  Define  Meteorology.     How  distinguished  from  Pneumatics  1 
What  different  subjects  does  Meteorology  lead  us  to  consider  1 


METEOROLOGY.  107 

finally,  in  the  relations  of  the  atmosphere  to  fiery  me- 
teors, as  thunder  and  lightning,  aurora  borealis,  and 
shooting  stars. 

SEC.  1.  Of  the  Extent,  Density,  and  Temperature  of 
the  Atmosphere. 

107.  The  atmosphere  is  a  thin  transparent  veil, 
enveloping  the  earth,  and  extending  to  an  uncertain 
height,  but  probably  not  less  than  one  hundred  miles 
above  it.  Since  air  is  elastic,  and  the  lower  portions 
next  to  the  earth  sustain  the  weight  of  the  whole  body 
of  air  above  them,  they  are  compressed  by  the  load,  as 
air  would  be  under  any  other  weight.  As  we  ascend 
above  the  earth,  the  air  grows  thinner  and  thinner  very 
fast,  so  that  if  we  could  rise  to  the  height  of  seven 
miles  in  a  balloon,  we  should  find  the  air  four  times 
as  rare  there  as  at  the  surface  of  the  earth.  The  air 
is,  indeed,  much  more  rare  on  the  tops  of  high  moun- 
tains than  at  the  level  of  the  sea  ;  and  at  a  height 
much  greater  than  that  of  the  highest  mountains  on 
the  globe,  man  could  not  breathe,  nor  birds  fly.  The 
upper  regions  of  the  atmosphere  are  also  very  cold. 
As  we  ascend  high  mountains,  even  in  the  torrid  zone, 
the  cold  increases,  until  we  finally  reach  a  point  where 
water  freezes.  This  is  called  the  term  of  congelation. 
At  the  equator,  it  is  about  three  miles  high  ;  but  in 
the  latitude  of  40,  it  is  less  than  two  miles,  and  in  the 
latitude  of  80,  it  is  only  one  hundred  and  twenty  fe.et 
high.  Above  the  term  of  congelation,  the  cold  con- 
tinues to  increase  till  it  becomes  exceedingly  intense. 
The  clouds  generally  float  below  the  term  of  con- 
gelation. Mountains,  when  very  high,  are  usually 
covered  with  snow  all  the  year  round,  even  in  the 

107.  Give  a  general  description  of  the  atmosphere,  as  to  its  height 
— density  at  different  herghts— cold  of  the  upper  regions.  What  is  the 
term  of  congelation  1  How  high  at  the  equator  \  At  40°  and  fcO°  7 


108  NATURAL   PHILOSOPHY. 

warmest  countries,  merely  because  they  are  above  this 
boundary. 

SEC.   2.     Of  the  Relations  of  the  Atmosphere  to 
Water. 

108.  Besides  common  air,  the  atmosphere  always 
contains  more  or  less  watery  vapor,  a  minute  portion  of 
fixed  air,  or  carbonic  acid,  and  various  exhalations, 
which  are  generally  too  subtile  to  be  collected  in  a 
separate  state.     By  the  heat  of  the  sun,  the  waters  on 
the  surface  of  the  earth  are  daily  sending  into  the 
atmosphere  vast  quantities  of  watery  vapor,  which  rises 
not  only  from  seas  and  lakes,  but  even  from  the  land, 
wherever  there   is   any  moisture.      The  vapor  thus 
raised,  either  mixes  with  the  air  and  remains  invisible, , 
or  it  rises  to  the  higher  and  colder  regions,  and  isj 
condensed  into  clouds.     Sometimes  accidental  causes 
operate  to  cool  it  near  the  surface  of  the  earth,  and! 
then  it  forms  fogs.    It  returns  to  the  earth  in  the  forms! 
of  dew,  and  rain,  and  snow,  and  hail. 

109.  Dew  does  not  fall  from  the  sky,  but  is  deposited' 
from  the  air  on  cold  surfaces,  just  as  the  film  of  moisture^ 
is,  which  we  observe  on  a  tumbler  of  cold  water  in  aJ 
sultry  day.      Here,  the  air  coming  in  contact  with! 
a  surface  colder   than   itself,   has   a   portion  of  thel 
invisible  vapor  contained  in  it  condensed  into  water.'.' 
In  the  same  manner,  on  clear  and  still  nights,  which 
are   peculiarly   favorable   to   the   formation   of  dew, 
the   ground    becomes    colder   than   the    air,    and   the 
latter  circulating  over  it,   deposits  on  it  and   on  all 
things  near  it,  a  portion  of  its  moisture.     Dew  does 
not  form  on  all  substances  alike  that  are  equally  ex- 

108.  What  other  elastic  fluids  besides  air  does  the  atmosphere  con- 
tain 1  "Whence  is  the  watery  vapor  derived  1  What  becomes  of  it  1 

109.  How  is  dew  formed  1  Does  dew  form  on  all  substances  alike  1 
What  receive  the  most  1    What  receive  none  1 


METEOROLOGY.  109 

posed  to  it.  Some  substances  on  the  surface  of  the 
earth  are  found  to  grow  colder  than  others,  and  these 
receive  the  greatest  deposit  of  dew.  Deep  water,  as 
that  of  the  ocean,  does  not  grow  at  all  colder  in  a 
single  night,  and  therefore  receives  no  dew ;  and  the 
naked  skins  of  animals,  being  warmer  than  the  air, 
receive  none ;  although  the  moisture  which  is  con- 
stantly exhaled  from  the  animal  system  itself,  as  soon 
as  it  comes  into  contact  with  the  colder  air  that  sur- 
rounds the  person,  may  be  condensed,  and  moisten  the 
skin  or  the  clothes  in  such  a  way  as  to  give  the  appear, 
ance  of  dew.  In  this  manner,  also,  frost  (which  is 
nothing  more  than  frozen  dew)  collects,  in  cold  weather, 
on  the  bodies  of  domestic  animals.  By  a  beautiful 
provision  of  Providence,  dew  is  always  guided  with  a 
frugal  hand  to  those  objects  which  are  most  benefited 
by  it.  Green  vegetables  receive  much  more  than  na- 
ked sand  equally  exposed,  and  none  is  squandered  on 
the  ocean. 

110.  Rain  is  formed  in  the  atmosphere  at  some 
distance  above  the  earth,  where  warm  air  becomes 
cooled.  If  it  is  only  cooled  a  few  degrees,  the  moist- 
ure may  merely  be  condensed  into  cloud ;  but  if  the 
cooling  is  greater,  rain  may  result ;  and  when  a  hot 
portion  of  air,  containing,  as  such  air  does,  a  great 
quantity  of  watery  vapor  in  the  invisible  state,  is 
suddenly  cooled  by  any  cause,  the  rain  is  more  abun- 
dant, or  even  violent.  In  such  cases,  it  may  have 
been  cooled  by  meeting  with  a  portion  of  colder  air, 
as  when  a  warm  southwesterly  wind  meets  a  cold 
northwester,  or  by  rising  into  the  upper  regions  near 
the  term  of  congelation.  In  some  parts  of  the  earth, 
as  in  Egypt,  and  in  a  part  of  Chili  and  Peru,  it  sel- 
dom or  never  rains,  for  there  the  winds  usually  blow 


110.  Where  is  rain  formed,  and  how  1   When  is  the  precipitation 
in  the  form  of  cloud  1  When  of  rain  1  When  is  the  rain  violent  *  In 
10 


110  NATURAL  PHILOSOPHY. 

steadily  in  one  direction,  and  encounter  none  of  those 
mixtures  with  colder  air  which  form  rain.  In  some 
other  countries,  as  the  northeastern  part  of  South 
America,  the  rains  are  excessive ;  and  in  others,  as 
most  tropical  countries,  the  rains  are  periodical,  being 
very  copious  at  particular  periods  called  the  rainy 
seasons,  while  little  or  none  falls  during  the  other  parts 
of  the  year. 

111.  Snow  is  formed  from  vapor  crystallized  by 
cold  instead  of  uniting  in  drops.  By  this  means  it  is 
converted  into  a  light  downy  substance,  which  falls 
gently  upon  the  earth,  and  forms  a  covering  that  con- 
fines the  heat  of  the  earth,  and  furnishes  an  admirable 
defence  of  the  vegetable  kingdom,  during  winter,  in  se-i 
vere  climates.  In  cold  climates,  flakes  of  snow  consist 
of  regular  crystals,  presenting  many  curious  figures, 
which,  when  closely  inspected,  appear  very  beautiful. 
Nearly  a  hundred  distinct  forms  of  these  crystals  have 
been  particularly  described  by  voyagers  in  the  polar! 
seas,  specimens  of  which,  as  they  appear  under  the] 
magnifier,  are  exhibited  in  the  following  diagram. 


When  a  body  of  hot  air  becomes  suddenly  and? 
intensely  cooled,  the  watery  vapor  is  frozen  and 
forms  hail.  The  most  violent  hailstorms  are  formed 
by  whirlwinds,  which  carry  up  bodies  of  hot  air  far 
beyond  the  term  of  congelation,  where  the  drops  of 

what  different  ways  is  the  hot  air  cooled  ^  Where  does  it  never  rain  1« 
Why  1    Where  are  the  rains  excessive  1   Where  periodical  1 

111.  Snow,  how  formed  1  What  purpose  does  it  serve  1  In  what 
manner  does  it  crystallize,  and  in  how  many  different  forms  *?  When 
is  hail  formed  1  How  are  the  most  violent  hailstorms  formed  1  How 


METEOROLOGY.  Ill 

rain  are  frozen  into  hailstones,  and  these  being  sus- 
|  tained  for  some  time  by  the  upward  force  of  the  whirl- 
I  wind,  accumulate  occasionally  to  a  very  large  size. 
Hailstorms  are  chiefly  confined  to  the  temperate  zones, 
and  seldom  occur  either  in  the  torrid  or  the  frigid 
zone.  In  the  equatorial  regions,  the  term  of  congela- 
tion is  so  high,  that  the  hot  air  of  the  surface,  if  raised 
by  a  whirlwind,  would  seldom  rise  beyond  it ;  and  in 
the  polar  regions,  the  air  does  not  become  so  hot  as  is 
required  to  form  a  hailstorm. 

SEC.  3.     Of  the  Relations  of  the  Atmosphere  to  Heat. 

112.  It  is  chiefly  by  the  agency  of  heat,  that  air  is 
put  in  motion.  If  a  portion  of  air  is  heated  more  than 
the  surrounding  portions,  it  becomes  lighter,  rises,  arid 
the  surrounding  air  flows  in  to  restore  the  equilibrium  ; 
or  if  one  part  be  cooled  more  than  another,  it  contracts 
In  volume,  becomes  heavier,  and  flows  off  on  all  sides 
until  the  equilibrium  is  restored.  Thus  the  air  is  set 
in  motion  by  every  change  of  temperature ;  and  as 
such  changes  are  constantly  taking  place,  in  greater 
or  less  degrees,  the  atmosphere  is  seldom  at  rest  at 
any  one  place,  and  never  throughout  any  great  extent. 
The  most  familiar  example  we  have  of  the  effects  of 
heat  in  setting  air  in  motion,  is  in  the  draught  of  a 
chimney.  When  we  kindle  a  fire  in  a  fireplace,  or 
stove,  it  rarefies  the  air  of  the  chimney,  and  the  denser 
air  from  without  rushes  in  to  supply  the  equilibrium, 
carrying  the  smoke  along  with  it.  Smoke,  when 
cooled,  is  heavier  than  air,  and  tends  to  descend,  and  does 
descend  unless  borne  up  by  a  current  of  heated  air.  A 


•  do  hailstones  acquire  so  large  a  size  *  To  what  regions  are  hailstorms 
chielly  confined  1    Why  do  they  not  occur  in  the  torrid  and  frigid 

'!  zones  1 

112.  By  what  agent  is  air  put  in  motion  1    Describe  the  process. 
How  is  the  draught  of  a  chimney  caused  1  Why  does  smoke  ascend  1 


112  NATURAL  PHILOSOPHY. 

hot  current  of  air  in  a  chimney  is  cooled  much  more 
rapidly  when  the  materials  of  the  chimney  are  damp 
than  when  they  are  dry,  and  therefore  it  will  cool  much 
faster  in  a  wet  than  in  a  dry  atmosphere.  Hence, 
chimneys  are  apt  to  smoke  in  wet  weather.  It  is  es- 
sential to  a  good  draught,  that  the  inside  of  a  chimney 
should  be  smooth,  for  air  meets  with  great  resistance 
in  passing  over  rough  surfaces.  Burning  a  chimney 
improves  the  draught,  principally  by  lessening  the  fric- 
tion occasioned  by  the  soot.  In  stoves  for  burning  an- 
thracite coal,  it  is  important  to  the  draught,  that  no  air 
should  get  into  the  chimney  except  what  goes  through 
the  fire.  On  account  of  the  great  resistance  which  a 
thick  mass  of  anthracite  opposes  to  air,  this  will  not 
work  its  way  through  the  coal  if  it  can  get  into  thej 
chimney  by  any  easier  route.  Hence  the  pipes  which 
conduct  the  heated  air  from  a  stove  to  the  chimney, 
should  be  close,  especially  the  joint  where  the  pipe  en- 
ters the  chimney ;  and  care  should  be  taken,  that  there 
should  be  no  open  fireplace,  or  other  means  of  commu- 
nication, between  the  external  air  and  the  flue  with 
which  the  stove  is  connected. 

113.  It  is  important  to  health,  that  the  apartments; 
of  a  dwelling-house  should  be  well  ventilated.     This  is? 
especially   the    case   with   crowded    rooms,    such    as 
churches  and  schoolhouses.     Of  the  method  of  venti-3 
lating  churches,  a  beautiful  specimen  is  afforded  in  the  i 
Centre  church,  in  New  Haven.     In  the  middle  of  the^ 
ceiling,  over  the  body  of  the  church,  is  an  opening^ 
through  the  plastering,  which  presents  to  the  eye  nothing 
but  a  large  circular  ornament  in  stucco.     Over  this, 
in  the    garret  of  the    building,    a  circular  enclosure 
of  wood  is  constructed,  on  the  top  of  which  is  built  a 

Why  do  chimneys  smoke  in  wet  weather  1  Why  should  a  chimney 
be  smooth  1  Why  does  burning  a  chimney  improve  the  draught  1 
What  precautions  are  necessary  in  burning  anthracite  coal,  in  order 
to  secure  a  good  draught  1 


METEOROLOGY.  113 

large  wooden  chimney,  leading  off,  at  a  small  rise,  to 
the  end  of  the  building,  where  it  enters  the  steeple. 
An  upper  window  of  the  steeple  being  open,  in  warm 
weather,  the  current  sets  upward  from  the  church  into 
the  chimney?  and  thence  into  the  tower,  and  completely 
i  ventilates  the  apartment  below.  A  door,  so  hung  as 
to  be  easily  raised  or  lowered  by  a  string,  leading  to 
a  convenient  place  at  the  entrance  of  the  church,  can 
be  opened  or  closed  at  pleasure.  In  cold  weather,  it 
will  generally  be  found  expedient  to  keep  it  closed, 
to  cut  off  cold  air,  opening  it  only  occasionally.  A 
schoolhouse  may  easily  be  ventilated  by  a  similar  con- 
trivance connected  with  a  belfry  over  the  center,  as  is 
done  in  several  schoolhouses  recently  built  in  New 
England. 

114.  Nature,  however,  produces  movements  of  the 
atmosphere  on  a  far  grander  scale,  in  the  form  of 
Winds.  These  are  exhibited  in  the  various  forms  of 
breezes,  high  winds,  hurricanes,  gales,  and  tornadoes ; 
varieties  depending  chiefly  on  the  different  velocities 
with  which  the  wind  blows.  A  velocity  of  twelve 
miles  an  hour  makes  a  strong  breeze  ;  sixty  miles,  a 
high  wind,  one  hundred  miles,  a  hurricane.  In  some 
extreme  cases,  the  velocity  has  been  estimated  as  high 
as  three  hundred  miles  an  hour.  The  force  of  the 
wind  is  proportioned  to  the  square  of  the  velocity  ;  a 
speed  ten  times  as  great  increases  the  force  a  hundred 
times.  Hence,  the  power  of  violent  gales  is  irresist- 
ible.  Air,  when  set  in  motion,  either  on  a  small  or 
on  a  great  scale,  has  a  strong  tendency  to  a  whirl- 
ing  motion,  and  seldom  moves  forward  in  a  straight 
line.  The  great  gales  of  the  ocean,  and  the  smal. 


113.  Ventilation,  in  what  cases  is  it  important  1    How  effected  'n 
churches — how  in  schoolhouses  1 

114.  Specify  the  different  varieties  of  winds.     State  the  velocity.  or 
a  breeze — of  a  high  wind — of  a  hurricane.  How  is  the  Ibrce  of  a  wind 
proportioned  to  tEe  velocity  1    Tendency  01  air  to  a  whirling  motion 

10* 


114  NATURAL   PHILOSOPHY. 

tornadoes  of  the  land,  often,  if  not  always,  exhibit 
more  or  less  of  a  rotary  motion,  and  sometimes  appear 
to  spin  like  a  top  around  a  perpendicular  axis,  at  the 
same  time  that  they  advance  forward  in  some  great 
circuit. 

115.  METEOROLOGICAL    INSTRUMENTS. — The    prin- 
cipal  of  these  are  the    Thermometer,  the  Barometer, 
and  the  Rain  Gage.     The  principle,  construction,  and 
uses  of  the  Barometer,  have  already  been  pointed  out, 
(Arts.  96  and  97.)     Since  it  informs  us  of  the  changes 
that  take  place  in  the  weight  and  pressure  of  the  at- 
mosphere, at  any  given  place,  on  which  depend  most 
of  the  changes  of  weather,  it  becomes  of  great  aid  in 
the  study  of  Meteorology,  and   has,  in   fact,  led  to  the 
knowledge  of  most  of  the   laws  of  atmospheric   phe- 
nomena   hitherto    established.     We    should,    in    pur- 
chasing, be  careful  to  select  an  instrument  of  good 
workmanship,   for   no  other  is  worthy  of  confidence. 
We  should  suspend  it  in  some  place  where  there  is  a 
free  circulation  of  air — as  in  an  open  hall,  having  an 
outside  door — and  we  should  take  the  exact  height  of 
the  mercury  at  the  times  directed  below  for  recording 
the  thermometrical  observations.     In  case  the  barome- 
ter is  falling  or  rising  with  unusual  rapidity,  observa- 
tions should  be  recorded  every  hour,  or  even  oftener, 
as  such  observations  afford  valuable  means  of  com- 
parison of  the   states  of  the  atmosphere  at  different 
places. 

116.  The  Thermometer  is  an  instrument  used   for 
measuring  variations  of  temperature   by  its  effects  on 
the  height  of  a  column  of  fluid.     As  heat  expands  and 
cold  contracts  all   bodies,  the  amount  of  expansion  or 
contraction  in  any  given  case,  is  made  a  criterion  of 


115.  What  are   the  three  leading  meteorological  instruments  ? 
Great  value  of  the  barometer.    Rules  for  selecting  a  barometer  and 
for  observing. 

116.  For  what  is  the  thermometer  used  7    What  shows  the  change 


METEOROLOGY.  115 

1  the  change  of  temperature.     Fahrenheit's  thermome- 
j  ter,  the  one  in  common  use,  consists  of  a  small  glass 
1  tube,  called  the  stem,  with  a  bulb  at  one  end,  and  a 
scale  at  the  side.     The  bulb  and  a  certain  part  of  the 
stem  are  filled  with  mercury.     The  scale  is  divided 
into  degrees  and  aliquot  parts  of  a  degree.     If  we  dip 
the  thermometer  into   boiling  water,  the  mercury  will 
expand  and  rise  in  the  stem  to  a  certain  height,  and 
there  remain  stationary.     We   will,  therefore,   mark 
that  point  on  the  stem,  and  then  transfer  the  thermome- 
ter to  a  vessel  where  water  is  freezing.     The  mercury 
now  descends  to  a  certain  level,  and  remains  there  sta- 
|  tionary,  as  before.     We  mark  this  point,  and  we  thus 
{  obtain  the  two  most  important  fixed  points  on  the  scale, 
|  namely,  the  freezing  and  boiling  points  of  water.     We 
I  will  now  apply  the  scale,  and  transfer  these  marks  from 
I  the  stem  to  the  scale,  and  divide  the  part  of  the  scale 
j  between  them  into  180  equal  parts,  continuing  the  same 
I  divisions  below  the  freezing  point  32  degrees,  where 
1  we  make  the  zero  point,  and  there  begin  the  graduation 
I  from  0  to  32,  the  freezing  point,  and  so  on  180  degrees 
j  more,  to  212,  the  boiling  point. 

The  best  times  for  making  and  recording  observa- 
I  tions,  are  when  the  mercury  is  lowest,  which  occurs 
i]  about  sunrise,  and  when  it  is  highest,  which  is  near 
1  two  o'clock  in  winter,  and  three  in  summer.  The  sum 
1  of  these  observations,  divided  by  two,  gives  the  aver- 
j  age,  or  mean,  for  the  twenty-four  hours ;  the  sum  of 
,  the  daily  means  for  the  days  of  a  month,  gives  the  mean 
1  for  that  month  ;  and  the  monthly  averages,  divided  by 
I  twelve,  give  the  annual  mean.  By  such  observations, 
*  any  one  may  determine  the  temperature  of  the  place 
I  where  he'^esides. 

]  of  temperature  1  Describe  Fahrenheit's  thermometer.  How  do  we 
ascertain  the  boiling  and  freezing  points  of  water  1  Into  how  many 
\  degrees  is  the  space  between  them  divided  1  Where  is  the  zero 
i  point,  and  at  what  degrees  are  the  freezing  and  boiling  points'! 
j  How  to  find  the  daily,  monthly,  and  annual  means  1 


116 


NATURAL   PHILOSOPHY. 


Fig.  55. 


117.  The  climate  of  the  United  States  is  very  va- 
riable, and  the  annual  range  of  the  thermometer  is 
greater  than  in  most  other  countries.  It  embraces 
140°,  extending  from  40°  below  zero,  (usually  marked 
— 40°,)  to  100°  above.  In  the  southern  part  of  New 
England,  the  mercury  seldom  rises  above  90,°  and  de- 
scends but  a  few  times  in  the  winter  below  zero.  From 
70°  to  80°  is  a  moderate  summer  heat.  Although  the 
equatorial  regions  of  the  earth  are,  in  general,  hotter 
than  places  either  north  or  south,  yet  we  have  seen 
that  the  temperature  of  a  place  depends  on  various 
other  circumstances,  as  well  as  on  the  latitude.  (Arts. 
82  and  107.) 

118.  The  Rain  Gage  is  an  instrument 
employed  for  ascertaining  the  amount  of 
water  that  falls  from  the  sky,  in  the  various 
forms  of  rain,  snow,  and  hail.  The  sim- 
plest form  is  a  tall  tin  cylinder,  with  a  fun- 
nel-shaped top,  having  a  graduated  glass 
tube  communicating  with  the  bottom,  and 
rising  on  the  side.  The  water  will  stand 
,  at  the  same  level  in  the  tube  and  in  the  cy- 
Minder,  and  the  divisions  of  the  tube  may 
Pbe  such  as  to  indicate  minute  parts  of  an 
inch,  and  thus  determine  the  depth  of  rain  that  falls  on 
the  area  of  the  funnel,  suppose  a  square  foot.  After 
the  rain  is  over,  the  water  may  be  removed  by  means 
of  the  stop-cock,  and  the  apparatus  will  be  ready  for 
a  new  observation.  It  is  useful  to  know  the  amount 
of  rain  that  falls  annually  at  any  given  place,  not  only 
in  reference  to  a  knowledge  of  the  climate,  but  also 
for  many  practical  purposes  to  which  water  is  applied, 

117.  What  is  said  of  the  climate  of  the  United  States  1    What 
is  the  annual  range  of  the  thermometer  1   In  New  England,  what  is 
the  range  *?    What  is  a  moderate  summer  heat  1 

118.  What  is  the  Rain  Gage  *?    Explain  the  simplest  form.    How 
to  find  the  amount  of  rain  fallen  1    Why  is  it  useful  to  know  the 
amount  of  rain  that  falls  1 


METEOROLOGY.  117 

such  as  feeding  canals,  turning  machinery,  or  irriga- 
ting land. 

SEC.  4.  Of  the  Relations  of  the  Atmosphere  to  Fiery 
Meteors. 

119.  The  luminous  phenomena  which  go  under  the 
general  name  of  "  fiery  meteors,"  are  Thunder  Storms, 
Aurora    Borealis,    and   Shooting   Stars.     Sudden  and 
violent  showers  of  rain,  in  hot  weather,  are  usually 
accompanied  by  thunder  and  lig  'tning.      The    light- 
ning is  owing  to  the  sudden  discharge  of  electricity, 
and  the  thunder  is  ascribed  to  the  rushing  together  of 
the  opposite  portions  of  air,  that   are  divided  by  the 
passage  of  the  electric  current.     The  snapping  of  a 
whip  depends  on  the  same  principle  as  a,  clap  of  thun- 
der.    The  lash  divides  the  air,  and  the  forcible  meet- 
ing of  the  opposite    parts  to  restore  the  equilibrium, 
produces  the  sound.     Whenever  hot  vapor  is  rapidly 
condensed,  a  great  amount  of  electricity  is  extricated. 
This  accumulates  in  the  cloud,  until  it  acquires  force 
enough  to  leap  from  that  to  some  other  cloud,  or  to  the 
earth,  or  to  some  object  near  it,  and  thus  the  explo- 
sion takes  place. 

120.  The  Aurora  Borealis,  or  Northern  Lights,  are 
most  remarkable  in  the  polar  regions,  and  are  seldom 
or  never  seen  in  the  torrid    zone.     They    sometimes 
present   merely   the    appearance  of  a  twilight  in  the 
north;  sometimes  they  shoot  up  in  streamers,  or  ex- 
hibit a  flickering  light,  called  Merry  Dancers  ;  some- 
times  they   span  the  sky  with    luminous   arches,    or 
bands ;  and  more  rarely  they  form  a  circle  with  stream- 

119.  What  are  the  three  varieties  of  fiery  meteors  ?    How   is 
lightning  produced  1    To  what  is  thunder  ascribed  1    How  explain- 
ed by  the  snapping  of  a  whip  1     Origin  of  the  electricity  of  thun- 

%<der  storms  1   When  does  an  explosion  take  place  1 

120.  Aurora  Borealis,  where  most  remarkable  1    Specify  the  sev- 


118  NATURAL  PHILOSOPHY. 

ers  radiating  on  all  sides  of  it,  a  little  southeast  of  tho 
zenith,  called  the  corona.     The  aurora  borealis  is  not 
equally  prevalent  in  all  ages,  but  has  particular  periods 
of  visitation,    after   intervals   of  many   years.     It  is 
more  prevalent  in  the  autumnal  months  than  the  other! 
parts  of  the  year,  and  usually  is  most  striking  in  the  | 
earlier  parts  of  the  night,  frequently  kindling  up  with  ] 
great  splendor  about  11  o'clock.     From  1827  to  1842, 
inclusive,  was  a  remarkable  period  of  auroras.     The  \ 
cause  of  this  phenomenon  is  not  known  ;  it  has  been 
erroneously  ascribed  to  electricity,  or  magnetism ;  but  | 
it  is  probably  derived  from  matter  found  in  the  plan- 
etary spaces,  with  which  the  earth  falls  in  while  it  is 
revolving  around  the  sun. 

121.  Shooting  Stars  are  fire-balls  which  fall  from 
the  sky,  appearing  suddenly,  moving  with  prodigious 
velocity,  and  as  suddenly  disappearing,  sometimes 
leaving  after  them  a  long  train  of  light.  They  are 
occasionally  observed  in  great  numbers,  forming  what 
are  called  Meteoric  Showers.  Two  periods  of  the  year 
are  particularly  remarkable  for  these  displays,  namely, 
the  9th  or  10th  of  August,  and  the  13th  or  14th  of 
November.  The  most  celebrated  of  these  showers 
occurred  on  the  morning  of  the  13th  of  November, 
1833,  when  meteors  of  various  sizes  and  degrees ; 
of  splendor,  descended  with  such  frequency  as  to 
give  the  impression  that  the  stars  were  all  falling 
from  the  firmament.  The  exhibition  was  nearly : 
equally  brilliant  in  all  parts  of  North  America,  and  \ 
lasted  from  about  11  o'clock  in  the  evening  till  sunrise. 
This  phenomenon  began  to  appear  in  some  parts  of 
the  world,  as  early  as  November,  1830,  and  increased 


eral  varieties.  Is  it  equally  prevalent  in  all  ages  1  What  was  a  re- 
markable period  1  Is  its  cause  known  "\  To  what  has  it  been  as- 
cribed *?  In  what  part  of  the  year  is  it  most  frequent  1 

121.  What  are  shooting  stars  1    What  two  periods  of  the  year  are 
remarkable  for  their  occurrence  1    When  did  the  greatest  meteoric 


ACOUSTICS,  119 

I  in  splendor  at  the  same  period  of  the  year,  every  year, 
;  until   1833,  when  it  reached  its  greatest  height.     It 
(was  repeated  on    a   smaller  scale,   every  year,  until 
1 1838,  since  which  time  nothing  remarkable  has  been 
'  observed  at   this   period.       The   meteoric  shower   of 
August  still  (1843)  continues.     Meteoric  showers  ap- 
pear to  rise    from  portions  of  a    body    resembling   a 
comet,  which  revolves  about  the  sun,  and  sometimes 
comes  so  near  the  earth  that  portions  of  it  are  attracted 
down  to  the  earth,  and  are  set  on  fire  as  they  pass 
through  the  atmosphere. 


CHAPTER  VI. 
ACOUSTICS. 

VIBRATORY    MOTION VELOCITY  OF   SOUND REFLEXION  OF  SOUND— 

MUSICAL  SOUNDS ACOUSTIC  TUBES STETHOSCOPE. 

122.  ACOUSTICS  (a  term  derived  from  a  Greek  word 
which  signifies  to  hear)  is  that  branch  of  Natural  Phi- 
losophy which  treats  of  Sound.  Sound  is  produced  by 
the  vibrations  of  the  particles  of  a  sounding  body. 
These  vibrations  are  communicated  to  the  air,  and 
by  that  to  the  ear,  which  is  furnished  with  a  curious 
apparatus  specially  adapted  to  receive  them  and  con- 
vey them  to  the  brain,  and  thus  is  excited  the  sen- 
sation of  hearing.  Vibration  consists  in  a  motion 
of  the  particles  of  a  body,  backward  and  forward, 
through  an  exceedingly  minute  space.  The  particles 
of  air  in  contact  with  the  body,  receive  a  correspond- 
ing motion,  each  particle  impels  one  before  it,  and  re- 
shower  occur  1  Describe  this  shower.  Whence  do  meteoric  show- 
ers arise  1 

122.  Define  Acoustics.  How  is  sound  produced  1  In  what  does 
vibration  consist  1  Does  it  imply  a  progressive  motion  1  What  bo- 


120  NATURAL   PHILOSOPHY. 

bounds,  and  thus  the  motion  is  propagated  from  parti- 
cle to  particle,  from  the  sounding  body  to  the  ear. 
Such  a  vibratory  motion  of  the  medium,  does  not  im- 
ply any  current  or  progressive  motion  in  the  medium 
itself,  but  each  particle  recovers  its  original  situation 
when  the  impulse  that  produced  its  vibration  ceases. 
Elastic  bodies  being  most  susceptible  of  this  vibratory 
motion,  are  those  which  are  usually  concerned  in  the 
production  of  sound.  Such  are  thin  pieces  of  board, 
as  in  the  violin  ;  a  steel  spring,  as  in  the  Jewsharp  ;  a 
glass  vessel,  and  cords  closely  stretched ;  or  a  column 
of  confined  air,  .as  in  wind  instruments.  If  we  stretch 
a  fine  string  between  two  fixed  points,  and  draw  it  out 
of  a  straight  line  to  A,  and  then  let  it  go,  it  will  pro- 
ceed to  nearly  the  same  distance  on  the  other  side,  to  * 


E,  whence  it  will  return  to  B,  and  thus  continue  to 
vibrate  through  smaller  and  smaller  spaces,  until  it 
comes  to  a  state  of  rest.  When  we  throw  a  stone 
upon  a  smooth  surface  of  water,  a  circle  is  raised  im- 
mediately around  the  stone  ;  that  raises  'another  circle 
next  to  it,  and  this  another  beyond  it,  and  thus  the 
original  impulse  is  transmitted  on  every  side.  This 
example  may  give  some  idea  of  the  manner  in  which 
sound  is  propagated  through  the  air  in  all  directions 
from  the  sounding  body. 

dies  are  most  susceptible  of  vibration  1  Give  examples.  Describe 
Fig.  56.  What  takes  place  Tvhen  a  stone  is  thrown  on  water  1 


ACOUSTICS.  121 

123.  Although  air  is  the  usual  medium  of  sound, 
yet  it  is  not  the  only  medium.      Solids  and  liquids, 
when  they  form  a  direct  communication  between  the 
sounding  body  and  the  ear,  conduct  sound  far  better 
than  air.      When  a  tea-kettle  is  near  boiling,  if  we 
apply  one  end  of  an  iron  poker  to  the  kettle,  and  put 
the  other  end  to  the  ear,  we  may  perceive  when  the 
water  begins  to  boil,  long  before  it  gives  the  usual  signs. 
If  we  attacn  a  string  to  the  head  of  a  fire-shovel,  and 
winding  the  ends  around  the  fore  fingers  of  both  hands, 
apply  them   to   the   ears,  and   then  ding  the  shovel 
against  an  andiron,  or  any  similar  object,  a  sound  will 
be  heard  like  that  of  a  heavy  bell.     The  ticking  of  a 
watch  may  be  heard  at  the  remote  end  of  a  long  pole, 

jor  beam,  when  the  ear  is  applied  to  the  other  end  ; 
;;and  if  the  watch  is  let  down  into  water,  its  beats  are 
.distinctly  heard  by  an  ear  placed  at  the  surface.  A 
ibell  struck  beneath  the  water  of  a  lake,  has  been 
ijheard  at  the  distance  of  nine  miles.  Air  is  a  better 
ijconductor  of  sound  when  moist  than  when  dry.  Thus, 
we  hear  a  distant  bell  or  a  waterfall  with  unusual 
•distinctness  just  before  a  rain,  and  better  by  night  than 
•by  day.  Air  conducts  sound  better  when  condensed, 
land  worse  when  rarefied.  On  the  tops  of  some  of  the 
Ihigh  mountains  of  the  Alps,  where  the  air  is  much 
irarefied,  the  sound  of  a  pistol  is  like  that  of  a  pop-gun. 

124.  The  velocity  of  sound  in  air  is  1130  feet  in  a 
.second,  or  a  little  more  than  a  mile  in  five  seconds. 
;On  this  principle,  we  may  estimate  the  distance  of  a 
thunder-cloud,  by  the  interval  between  the  flash  and 

ithe  report.  For  example,  an  interval  of  five  seconds, 
*  gives  1130x5=5650  feet,  or  a  little  more  than  a  mile. 
'A  feeble  sound  moves  just  as  fast  as  a  loud  one.  Its 


123.  Is  air  the  only  medium  of  sound  *!     Conducting  power  of 
solids  and  liquids  1  Experiment  with  a  tea-kettle— with  a  fire-shovel 
,— with  a  watch.    Conduct! no;  power  of  moist  air/?— Of  rarefied  air  1 

124.  Velocity  of  sound.  How  to  estimate  the  distance  of  a  thunder 

11 


122  NATURAL   PHILOSOPHY. 

velocity  is  not  altered  by  a  high  wind  in  a  direction  at 
right  angles  to  the  course  of  the  wind  ;  but  in  the 
same  direction,  the  comparatively  small  velocity  of  the 
wind  is  to  be  added,  and  in  the  opposite  direction  to  be 
subtracted.  In  water,  the  velocity  of  sound  is  about 
four  times  as  great  as  in  air,  being  4709  feet  per 
second  ;  and  in  cast  iron  its  velocity  is  more  than  ten, 
times  as  great  as  in  air,  being  no  less  than  11,895  feet 
per  second. 

125.  Sound  is  capable  of  being  reflected,  and  is  thus 
sometimes  returned  to  the  ear,  forming  an  eclw.    Thus, 
the  sound  of  the  human  voice  is  sometimes  returned 
to  the  speaker,  or  other  persons  near  him,  in  a  repeti- 
tion usually  somewhat  feebler  than  the  original  sound ; 
but  it  may  be  louder  than  that,  if  several  reflected  \ 
waves  are  unitedly  conveyed  to  the  ear.     When  one  ] 
stands   in   the  centre  of  a   hollow  sphere   or   dome,  j 
numerous   waves    being   reflected    from   the   concave 
surface  so  as  to  meet  in  the  centre,  a  sound  originally  I 
feeble  becomes  so  augmented  as  to  be  astounding.     A I 
cannon  discharged  among  hills  or  mountains,  reverbe-| 
rates  in  consequence  of  the  repeated  reflexions  of  the! 
sound. 

126.  A  sound  becomes  musical  when  the  vibrations 
are   performed  with  a  certain  degree  of  frequency.! 
The  slow  flapping  of  the  wings  of  a  domestic  fowl  hasj 
nothing  musical  ;  but  the  rapid  vibration  of  the  wingsf 
of  a  humming-bird,  produces  a  pleasant  note.      The 
slow  falling  of  trees  before  a  high  wind,  is  attended 
with  a  disagreeable  crash  ;  the  rapid  prostration  of  the 
trees  of  a  forest  by  a  tornado,  with  a  sublime  roar. 
A  string  stretched  between  two  points,  and  made  to 

cloud  ?  Velocity  of  a  feeble  sound — effect  of  a  high  wind  1  Velocity 
of  sound  in  water  1 

125.  Echo,  how  produced — when  louder  than  the  original  sound  1 
Effect  of  a  dome — of  a  cannon  among  hills  1 

126.  How  a  sound  becomes  musical  V- examples  in  the  wings  of 
birds— in  falling  trees— in  a  vibrating  string.    How  does  increasing 


ACOUSTICS.  123 

vibrate  very  slowly,  has  nothing  musical ;  but  when 
the  tension  is  increased,  and  the  vibrations  quickened, 
the  note  grows  melodious.  The  strings  of  a  violin 
give  different  sounds  in  consequence  of  affording  vibra- 
tions more  or  less  rapid.  The  larger  strings,  having 
slower  vibrations,  afford  graver  notes.  The  screws 
enable  us  to  alter  the  degree  of  tension,  and  thus  to 
increase  or  diminish  the  number  of  vibrations  at  plea- 
sure  ;  and  by  applying  the  fingers  to  the  strings,  we 
can  shorten  them  more  or  less,  producing  sounds  more 
or  less  acute,  by  increasing  the  number  of  vibrations 
in  a  given  time.  In  wind  instruments,  as  the  flute,  the 
vibrating  body  which  produces  the  musical  tone  is  the 
column  of  air  included  within.  This,  by  the  impulse 
given  by  the  mouth,  is  made  to  vibrate  with  the  requisite 
frequency,  which  is  varied  by  opening  or  closing  the 
stops  with  the  fingers.  The  shorter  the  column,  the 
more  rapid  is  the  vibration,  and  the  more  acute  the 
sound  ;  and  the  length  of  the  vibrating  column  is 
determined  by  the  place  of  the  stop  that  is  opened,  the 
higher  stops  giving  sharper  sounds  because  the  vibrating 
columns  are  shorter.  The  pipes  of  an  organ  sound  on 
a  similar  principle,  the  wind  being  supplied  by  a  bellows 
instead  of  the  breath.  In  certain  instruments,  as  the 
clarinet  and  hautboy,  the  vibrations  are  first  commu- 
nicated from  the  lips  of  the  performer  to  a  reed,  and 
from  that  to  the  column  of  air. 

127.  Sounds  differing  from  each  other  by  certain 
intervals,  constitute  musical  notes.  The  singing  of 
birds  affords  sweet  sounds  but  no  music,  being  uttered 
continuously  and  not  at  intervals.  Man  only,  among 
animals,  has  the  power  of  uttering  sounds  in  this  man- 

the  tension,  the  size,  or  the  length  of  the  string,  affect  the  pitch  1 
Example  in  the  violin.  What  produces  the  musical  tone  in  wind 
instruments'?  Why  does  opening  or  closing  the  stops,  alter  the 
pitch  1  Explain  the  use  of  a  reed. 

127.  What  sounds  constitute  musical  notes'?   Why  is  not  the  sing- 
ing of  birds  music  1   Why  is  man  alone  capable  of  uttering  musical 


124  NATURAL  PHILOSOPHY. 

ner  ;  and  his  voice  alone,  therefore,  is  endued  with 
the  power  of  music.  Music  becomes  a  branch  of 
mathematical  science,  in  consequence  of  the  relation 
between  musical  notes,  and  the  number  of  vibrations 
that  produce  them  respectively.  Although  we  cannot 
say  that  one  sound  is  larger  than  another,  yet  we  can 
say  that  the  vibrations  necessary  to  produce  one  sound 
are  twice  or  thrice,  or  any  number  of  times,  more 
frequent  than  those  of  another  ;  and  the  number  of 
vibrations  necessary  to  produce  one  note  has  a  fixed 
ratio  to  the  number  which,  produces  another  note. 
Thus,  if  we  dimmish  the  length  of  a  musical  string  one 
half,  we  double  the  number  of  vibrations  in  a  given 
time,  and  it  gives  a  sound  eight  notes  higher  in  the 
scale  than  that  given  by  the  whole  string,  and  is  called 
an  octave.  Hence,  these  sounds  are  said  to  be  to  each 
other  in  the  ratio  of  2  to  1,  because  this  is  the  ratio  of 
the  numbers  of  vibrations  which  produce  them.  A 
succession  of  single  musical  sounds  constitutes  melody  ; 
the  combination  of  such  sounds,  at  proper  intervals, 
forms  chords  ;  and  a  succession  of  chords,  produces 
harmony.  Two  notes  formed  by  an  equal  number  of 
vibrations  in  a  given  time,  and  of  course  giving  the 
same  sound,  are  said  to  be  in  unison.  The  relation 
between  a  note  and  its  octave  is,  next  after  that  of  the 
unison,  the  most  perfect  in  nature  ;  and  when  the  two 
notes  are  sounded  at  the  same  time,  they  almost  entirely 
unite.  Chords  are  produced  by  frequent  coincidences 
of  vibration,  while  in  discords  such  coincidences  are 
more  rare.  Thus,  in  the  unison,  the  vibrations  are 
exactly  coincident ;  in  the  octave,  the  two  coincide 
at  the  end  of  every  vibration  of  the  longer  string, 
the  shorter  meanwhile  performing  just  two  vibra- 
tions ;  but  in  the  second,  the  vibrations  of  the  two 

sounds  1  How  does  music  become  a  branch  of  mathematical 
science  1  Example  in  a  musical  string.  Define  melody,  chords, 
harmony,  unison.  How  are  chords  produced  1  How  discords  1 


ACOUSTICS.  125 

I  strings  coincide  only  after  eight  of  one  string  and  nine 
of  the  other,  and  the  result  is  a  harsh  discord. 

128.  When  an  impulse  is  given  to  air  contained  in 
an  open  tube,  the  vibrations  coalesce,  and  are  propa- 
;  gated  farther  than  when  similar  impulses  are  made 
on  the  open  air.  Hence  the  increase  of  sound  effect- 
ed by  horns  and  trumpets,  and  especially  by  the  speak- 
ing trumpet.  Alexander  the  Great  is  said  to  have 
had  a  horn,  by  means  of  which  he  could  give  .orders 
to  his  whole  army  at  once.  Acoustic  Tubes  are  em- 
ployed for  communicating  between  different  parts  of 
a  large  establishment,  as  a  hotel,  or  manufactory,  by 
the  aid  of  which,  whatever  is  spoken  at  one  extremity  is 
heard  distinctly  at  the  other,  however  remote.  They 
are  usually  made  of  tin,  being  trumpet-shaped  at  each 
end.  They  act  on  the  same  principle  as  the  speaking 
trumpet.  The  Stethoscope  is  an  instrument  used  by 
physicians,  to  detect  and  examine  diseases  of  the  lungs 
and  the  heart.  It  consists  of  a  small  pipe  of  wood  or 
ivory  with  funnel-shaped  mouths,  one  of  which  is  ap- 
plied firmly  to  the  part  affected  and  the  other  to  the 
oar.  By  this  means  the  processes  that  are  going  on 
in  the  organs  of  respiration,  and  in  the  large  blood- 
vessels about  the  heart,  may  be  distinctly  heard. 

128.  Explain  the  effect  of  horns  and  trumpets.  Use  of  Acoustic 
Tubes.  How  made  1  Explain  the  construction  and  use  of  the 
Stethoscope.  / 

11* 


CHAPTER  VII. 
ELECTRICITY.* 

DEFINITIONS CONDUCTORS     AND      NON-CONDUCTORS — ATTRACTION* 

AND  REPULSIONS — ELECTRICAL  MACHINES — LEYDEN    JAR — ELEC- 
TRICAL   LIGHT    AND   HEAT THUNDER    STORMS — LIGHTNING    RODS 

EFFECTS  OF  ELECTRICITY  ON  ANIMALS. 

129.  MORE  than  two  thousand   years   ago,  Theo- 
phrastus,  a  Greek  naturalist,  wrote  of  a  substance  we 
call  amber,  which,  when  rubbed,  has  the  property  of 
attracting  light  bodies.     The  Greek  name   of  amber 
was  electron,  (TjXsx-r^ov,)  whence  the  science  was  de* 
nominated  ELECTRICITY.     The  inconsiderable    expert 
ment  mentioned  by  Theophrastus,  was  nearly  all  that 
the  ancients  knew  of  this  mysterious  agent ;  but  for 
two  or  tnree  centuries  past,  new  properties  have  been 
successively  discovered,  and  new  modes  of  accumu- 
lating it  devised,  until  it  has  become  one  of  the  most 
important  and  interesting  departments  of  natural  sci, 
ence.     It  is  common  to  call  this  power,  whatever  it 
is,  the  electric  fluid,  although    it  is  of  too   subtile  a 
nature  for  us  to  show  it,  as  we  do  air,  and  prove  that 
it  possesses  the  properties  of  ordinary  matter.     But  as 
it  is  more  like  an  elastic  fluid  of  extreme  rarity,  than 
like  any  thing  else  we  are  acquainted  with,  it  is  con- 
venient to  denominate  it  a  fluid,  although  we  know  very 
little  of  its  nature. 

130.  Some  bodies  permit  the  electric  fluid  to  pass 
freely  through  them,  and  are  hence  called  conductors  / 
others  hardly  permit  it  to  pass  through  them  at  all,  and 

*  The  experiments  in  this  chapter  are  so  simple,  and  require  so  little  appa-^ 
ratus,  that  it  is  hoped  the  learner  will  generally  have  the  advantage  of  wit- 
nessing them,  which  will  add  much  more  than  mere  description  to  his  im- 
provement and  gratification. 

129.  Explain  the  name  electricity.    What  did  the  ancients  know 
of  this  science  1    Its  progress  within  two  hundred  years  1    Why  is. 
electricity  called  a  fluid  1 

130.  Define  conductors  and  non-conductors.    Give  examples  of 


ELECTRICITY.  127 

are  therefore   called  non-conductors.     Metals  are  the 
;  Ibest  conductors ;  next,  water  and  all  moist  substances ; 
land  next,  the  bodies  of  animals.     Glass,  resinous  sub- 
ttances,  as  amber,  varnish,  and  sealing  wax ;  air,  silk, 
wool,  cotton,  hair,  and  feathers,    are   non-conductors. 
Wood,  stones,  and  earth,  hold  an  intermediate  place : 
they  are  bad  conductors  when  dry,  but  much  better 
when   moist ;    and  air   itself  has   its   non-conducting 
power  greatly  impaired  by  the  presence  of  moisture. 
Electricity  is  excited  by  friction.     If  I  rub  the  side  of 
;a  dry  glass  tumbler,  or  a  lamp  chimney,  on  my  coat 
sleeve,  the  electricity  excited  will  manifest  itself  by 
i attracting  such  light  substances  as  bits  of  paper,  cot- 
jton,  or  down.     A  stick  of  sealing-wax,  when  rubbed, 
iexhibits  similar  effects.     When  an  electrified  body  is 
^supported  by  non-conductors  so  that  its  electricity  can- 
:j*iot  escape,  it  is  said  to  be  insulated.     Thus,  a  lock  of 
(cotton  suspended  by  a  silk  thread  is  insulated,  because 
jif  electricity  be  imparted  to  the   cotton,  it  remains, 
':since   it  cannot  make  its  escape   either  through  the 
.'thread,  or  through  the  air,  both  being  non-conductors. 
|,A  brass  ball  supported  by  a  pillar  of  glass  is  insulated ; 
but  when  supported  on  a  pillar  of  iron  or  any  other 
i, metal,  it  is  uninsulated,  since  the  electricity  does  not 
remain  in  the  ball,  but  readily  makes  its  escape  through 
the  metallic  support.     By  knowing  how  to  avail  our- 
selves of  the  conducting  properties  of  some  substances, 
and  the  non-conducting  properties  of  other  substances, 
we  can  either  confine,  or  convey  off  the  electric  fluid 
at  pleasure. 

131.  There  are  a  number  of  different  classes  of 
.phenomena  which  electricity  exhibits ;  as  attraction 
and  repulsion — 4ieat  and  light — shocks  of  the  animal 
.system — and  mechanical  violence.  These  will  suc- 

each.  How  is  conducting  power  affected  by  moisture  1  How  is 
electricity  excited  1  When  is  a  body  insulated  1  Give  examples 
,of  insulation. 


128  NATURAL   PHILOSOPHY. 

cessively  claim  our  attention  ;  but  as  the  properties  of 
electricity  were  first  discovered  by  experiment,  so  it 
is  by  experiments,  chiefly,  that  they  are  still  to  be 
learned.  We  will  therefore  describe,  first,  a  few  such 
experiments  as  every  one  may  perform  for  himself, 
and  afterwards  such  as  require  the  aid  of  an  electrical 
machine. 

SEC.  1.  Of  Electrical  Attractions  and  Repulsions. 

132.  For  a  few  simple  experiments,  we  will  stretch  a 
wire  horizontally  between  the  opposite  walls  of  a  room, 
or  between  any  two  convenient  points,  as  represented 
in  figure  57.  This  will  afford  a  convenient  support 

Fig.  57. 


€> 

a 


for  electroscopes,  as  those  contrivances  are  called,  which , 
are  used  for  detecting  the  presence  and  examining  the 
properties  of  electricity.  A  downy  feather,  a  lock  of 
cotton,  or  pith-balls,*  are  severally  convenient  substan- 
ces for  electroscopes.  To  one  of  these,  say  a  pith- 
ball,  we  will  tie  a  fine  linen  thread,  about  nine  inches 
long,  and  suspend  it  from  the  wire,  as  at  a.  By 
slightly  wetting  the  thumb  and  finger  and  drawing  the 

*  The  pith  of  elder,  of  corn  stalk,  or  of  dry  stalks  of  the  artichoke,  is  suit- 
able for  this  purpose. 

131.  What  different  classes  of  phenomena  does  electricity  exhib- 
it 1    Use  of  experiments. 

132.  Describe  the  apparatus  in  Fig.  57.    How  is  the  tube  excited  1 


ELECTRICITY.  129 

thread  through  them,  it  becomes  a  good  conductor,  and 
the  electroscope  is  therefore   uninsulated.     We    will 
now  take  a  thick  glass  tube  and  rub  it  with  a  piece  of 
silk,  (or  a  dry  silk  handkerchief,)  by  which  means  the 
!  tube  will  be  excited,  and  on  approaching  it  towards 
!  the  electroscope,  the  pith-ball  will  be  attracted  towards 
!  it,  as  at  b,  and  may  be  led  in  any  direction  by  shifting 
the  position  of  the  tube  ;   or  if  the  tube  be  brought 
nearer,  the  ball  will  stick  fast  to  it.     We  will   next 
suspend  two  other  balls,  c  and  d,  by  silk  threads,  in 
!  which  case  they  will  be  insulated.     If  we  now  ap- 
proach the  excited  tube,  the  balls  will  first  be  attracted 
to  it,  but  as  soon  as  they  touch  it,  they  will  fly  off,  and 
the  tube  when  again  brought  towards  them  will  no 
longer  attract  but  will  repel  them,  and  they  will  mu- 
tually repel  each  other  as  in  the  figure ;  and  if  the 
lock  of  threads,  e,  be  electrified,  they  will  also  repel 
each  other.     A  stick  of  sealing-wax  excited  and  ap- 
plied to  the  electroscopes  will   produce  similar  effects, 
j  But  if  we  first  electrify  the  ball  with  glass,  and  then 
|  bring  near  it  the  sealing-wax,  previously  excited,  it  will 
j  not  repel  the  ball,  as  the  excited  tube  does,  but  will  first 
j  attract  it  as  though  it  were  unelectrified,  and  then  re- 
j  pel  it ;  and  now  the  excited  glass  tube  will  attract  it. 
j  Hence  it  appears  that  the  glass  and  the  sealing-wax, 
|  when  excited,  produce  opposite  effects :  what  one  at- 
!  tracts   the   other   repels.     Each   repels  its  own,   but 
I  attracts  the  opposite.     Glass  repels  a  body  electrified 
I  by  itself,  but  attracts  a  body  electrified  by  sealing-wax ; 
i  and  sealing-wax  repels  a  body  electrified  by  itself,  but 
L  attracts  a  body  electrified  by  glass.     In  the  figure,  h 
represents  two  balls  differently  electrified,  one  by  glass 
and  the  other  by  sealing-wax,  and  therefore  attracting 
\  each  other.     This  fact  has  led  to  the  conclusion,  that 

i.  Effect  when  applied  to  the  uninsulated  ball— to  the  insulated  balls— to 
i  the  threads.  Describe  the  effects  when  sealing-wax  is  used— when  the 
,  balls  are  differently  electrified.  What  are  the  two  kinds  of  electricity  1 


130  NATURAL  PHILOSOPHY. 

there  are  two  kinds  of  electricity  ;  one  excited  by  glass 
and  a  number  of  bodies  of  the  same  class,  called  the 
vitreous  electricity,  and  the  other  excited  by  sealing. 
wax  and  other  bodies  equally  numerous,  of  the  same 
class  with  it,  called  the  resinous  electricity.  Vitreous 
electricity,  is  sometimes  called  positive)  and  resinous 
electricity  negative. 

133.  The  foregoing   cases  of  electrical  attractions 
and  repulsions  constitute  important  laws  of  electrical 
action,  and  are  to  be  treasured  up  in  the  memory  in  the 
following  propositions : 

First.  An  electrified  body  attracts  all  unelectrified 
matter. 

Secondly.  Bodies  electrified  similarly,  that  is,  both 
positively  or  both  negatively,  repel  each  other. 

Thirdly.  Bodies  electrified  differently,  that  is,  one 
positively  and  the  other  negatively,  attract  each  other. 

Fourthly.  The  force  of  attraction  or  repulsion  is  in- 
versely  as  the  square  of  the  distance  ;  that  is,  when  two 
balls  are  electrified,  the  one  positively  and  the  other 
negatively,  the  force  of  attraction  increases  rapidly  as 
they  draw  near  to  each  other,  being  four  times  as  great 
when  twice  as  near,  and  a  hundred  times  as  great  when 
ten  times  as  near.  Repulsion  follows  the  same  law; 
that  is,  when  two  balls  are  similarly  electrified,  it  re- 
quires four  times  the  force  to  bring  them  twice  as  near 
to  each  other,  and  a  hundred  times  the  force  to  bring 
them  ten  times  as  near  as  before. 

SEC.  2.  Of  Electrical  Apparatus. 

134.  Electrical  machines  afford  the  means  of  accu- 
mulating the  electric   fluid,  so  as  to  render  its  effects 
far  more  striking  and  powerful  than  they  appear  in  the 
simple   experiments   already   recited.      The   cylinder 

133.  State  the  four  laws  of  electrical  attraction  and  repulsion 


ELECTRICITY. 


131 


.machine   is   represented   in    Fig.    58.     Its    principal 
i  parts  are  the  cylinder,  the  frame,  the  rubber,  and  the 

Fig.  58. 


prime  conductor.  The  cylinder  (A)  is  of  glass,  from 
eight  to  twelve  inches  in  diameter,  and  from  twelve 
to  eighteen  inches  long.  The  frame  (B  B)  is  made 
of  hard  wood,  dried  and  varnished.  The  rubber  (C) 
consists  of  a  leathern  cushion,  stuffed  with  hair  like 
the  pad  of  a  saddle.  This  is  covered  with  a  black  silk 
cloth,  having  a  flap,  which  extends  from  the  cushion 
over  the  top  of  the  cylinder  to  the  distance  of  an 
inch  from  the  points  of  the  prime  conductor,  to  be 
mentioned  presently.  The  rubber  is  coated  with  an 
amalgam,  composed  of  quicksilver,  zinc,  and  tin,  which 
preparation  has  been  found  by  experience  to  produce 


134.  Describe  the  electrical  machine— the  cylinder— the  frame— 
the  rubber— the  amalgam— the  prime  conductor. 


132  NATURAL   PHILOSOPHY. 

a  high  degree  of  electrical  excitement,  when  subjected 
to  the  friction  of  glass.  The  prime  conductor  (D)  is 
usually  a  hollow  cylinder  of  brass  or  tin,  with  rounded 
ends.  It  is  mounted  on  a  solid  glass  pillar,  (a  junk- 
bottle  with  a  long  neck  will  answer,)  with  a  broad  and 
heavy  foot  made  of  wood  to  keep  it  steady.  The  cyl- ; 
inder  is  perforated  with  small  holes,  for  the  reception 
of  wires  (c)  with  brass  knobs.  It  is  important  in  an- 
electrical machine,  that  the  work  should  be  smooth 
and  free  from  points  and  sharp  edges,  since  these  have- 
a  tendency  to  dissipate  the  fluid,  as  will  be  more  fully 
understood  hereafter.  For  a  similar  reason,  the  ma- 
chine should  be  kept  free  from  dust,  the  particles  of 
which  act  as  points,  and  dissipate  the  electricity. 

135.  By  the  friction  of  the  glass  cylinder  against 
the  rubber,  electricity  is  produced,  which  is  received* 
by  the  points,  and  thus  diffused  over  the  surface  of  the> 
prime  conductor,  and  may  be  drawn  from  it  by  the> 
knuckle,  or  any  conducting  substance.  In  order  to» 
indicate  the  degree  of  excitement  in  the  prime  con- 
ductor, the  Quadrant  Electrometer  is  attached  to  it,  as* 
is  represented  at  E,  Fig.  58.  This  electrometer  iss 
formed  of  a  semicircle,  usually  of  ivory,  divided  into- 
degrees  and  minutes,  from  0  to  180.  The  index  con- 
sists of  a  straw,  moving  on  the  center  of  the  disk,  and! 
carrying  at  the  other  extremity  a  small  pith-ball.  The- 
perpendicular  support  is  a  pillar  of  brass,  or  some  con-j 
ducting  substance.  When  this  instrument  is  in  a  per- 
pendicular position,  and  not  electrified,  the  index  hangs 
by  the  side  of  the  pillar,  perpendicularly  to  the  hori- 
zon; but  when  the  prime  conductor  is  electrified,  it 
imparts  the  same  kind  of  electricity  to  the  index,  re- 
pels it,  and  causes  it  to  rise  on  the  scale  towards  ai> 
angle  of  90  degrees,  which  point  indicates  a  full  charge- 


135.  How  is  the  electricity  produced  1  Describe  the  quadrant  elec- 
trometer, and  show  how  it  indicates  the  degree  of  the  charge. 


ELECTRICITY.  133 

136.  Let  us  now  try  a  few  experiments.  If  we 
(turn  the  machine  one  or  two  rounds,  the  prime  con- 
Muctor  will  be  charged,  and  the  quadrant  electrometer 
[will  remain  fixed  at  90  degrees.  We  will  first  exam- 
Sine  the  conducting  powers  of  different  bodies.  A  glass 
'tube  held  in  the  hand  and  applied  to  the  prime  con- 
ductor will  not  cause  the  index  of  the  electrometer  to 
fall,  because  glass  is  a  non-conductor  of  electricity ; 
but  an  iron  rod  thus  applied,  will  cause  the  index  to 
fall  instantly,  iron  being  a  good  conductor,  and  permit- 
ting the  fluid  readily  to  escape  first  to  my  hand,  and 
through  my  person  to  the  floor,  and  finally  to  the  earth. 
On  applying  a  knuckle  to  the  prime  conductor,  we 
find,  in  the  same  manner,  that  the  animal  system  is  a 
!good  conductor,  as  the  fluid  is  instantly  discharged  and 
ithe  index  falls.  On  the  other  hand,  a  piece  of  sealing- 
jwax  will  not  affect  the  index,  and  is  therefore  a  non- 
iconductor.  So,  if  we  hold  a  lock  of  cotton  by  a  silk 
jthread  it  will  scarcely  affect  the  electrometer,  while  if 
held  by  a  linen  thread,  the  fluid  will  be  drawn  off  and 
the  index  will  fall.  It  is  very  useful  for  the  learner 
to  try  in  this  way  the  conducting  powers  of  a  great 
variety  of  bodies.  Some  he  will  find  to  affect  the 
j electrometer  very  little,  and  he  will  thus  know  them 
!to  be  non-conductors  ;  others  will  instantly  cause  it 
!to  fall,  and  are  known  as  good  conductors.  Others 
'will  cause  the  index  to  descend  gradually,  and  are  of 
;  course  imperfect  conductors.  These  last,  on  being 
I  moistened  with  the  breath  or  wet  with  water,  will  in- 
dicate an  increase  of  conducting  power.  A  long  stick 
*  of  wood,  as  a  broom-handle,  will  be  found  to  conduct 
1  with  less  power  than  a  short  stick  of  the  same,  and  a 
large  thread  will  conduct  better  than  a  small  one. 


136.  Experiments  on  the  conducting  powers  of  bodies — glass— iron 
— the  knuckle — sealing-wax — a  silk  thread.    State  the  efl'ect  of  each 
,  of  these.     What  is  the  effect  on  conducting  power  produced  by 
moisture— by  increasing  the  length  or  size  of  a  bad  conductor  1 
12 


134 


NATURAL   PHILOSOPHY. 


Fig.  59. 


Thus  all  the  different  circumstances  affecting  the  con- 
ducting  power,  may  be  ascertained  ;  and  upon  the 
knowledge  of  these  relative  powers,  depends  the  art 
of  managing  the  electric  fluid,  whether  in  the  form  of 
common  electricity  or  in  that  of  lightning. 

137.  The  laws  of  attraction  and  repulsion  may  be 
verified  by  the  aid  of  an  electrical  machine,  much 
more  strikingly  than  by  the  simple  apparatus  men- 
domed  in  Articles  132  and  133.  If  we  hang  a  lock  of 
hair  to  the  prime  conductor,  on  turning  the  machine 
the  hairs  will  recede  violently  from  each  other,  be- 
cause bodies  similarly  electrified  repel  each  other. 
By  placing  light  bodies,  as  paper  images,  locks  of  cot- 
ton, or  light  feathers,  between  one  plate  connected  with 
the  prime  conductor  and  another  which  is  uninsulated, 
as  is  represented  in  figure  59,  (the  upper  plate  being- 
hung  to  the  prime  conductory) 
the  electrical  dance  may  be  per- 
formed. The  images  will  first 
be  attracted  to  the  upper  plate, 
but  instantly  imbibing  the  same 
electricity,  they  will  be  repelled 
by  the  upper  and  attracted  by 
the  lower  plate  •  on  descending 
to  the  latter,  they  will  give  up 
their  charge  and  return  again 
to  the  upper  plate  to  repeat  the  . 
process,  thus  performing  a  kind 
of  dance,  which  when  performed 
by  little  images  of  men  and 
women,  is  often  very  amusing. 
Most  electrical  machines  are  furnished  with  a  variety 
of  apparatus  for  illustrating  the  principles  of  electrical 
attractions  and  repulsions,  such  as  a  chime  of  bells, 
the  electrical  horse-race,  the  electrical  wind-mill,  and 

137.  Effect  when  a  lock  of  hair  is  hung  to  the  prime  conductor  ^ 
How  is  the  electrical  dance  performed  1 


ELECTRICITY. 


135 


the  like  ;  but  these  must  be  seen  in  order  to  be  fully 
understood,  and  therefore  their  exhibition  is  left  to  the 
instructor. 

138.  The  Leyden  Jar  is  a  piece  of  ap- 
paratus used  for  accumulating  a  large  Fig.  60. 
quantity  of  electricity.  It  consists  of  a 
glass  jar  coated  on  both  sides  with  tin  foil, 
except  a  space  .on  the  upper  end,  within 
two  or  three  inches  of  the  top,  which  is  either 
left  bare,  or  is  covered  with  a  coating  of  var- 
:  nish,  or  a  thin  layer  of  sealing  wax.  To 
the  mouth  of  the  jar  is  fitted  a  cover  of  hard 
j  baked  wood,  through  the  center  of  which 
!  passes  a  perpendicular  wire,  terminating 
I  above  in  a  knob,  and  below  in  a  fine  chain 
j  that  rests  on  the  bottom  of  the  jar.  On  presenting  the 
knob  of  the  jar  near  the  prime  conductor  of  an  elec- 
trical machine,  while  the  latter  is  in  operation,  a  series 
of  sparks  pass  between  the  conductor  and  the  jar, 
which  will  gradually  become  more  and  more  feeble, 
until  they  cease  altogether.  The  jar  is  then  said  to 
be  charged.  If  we  now  take  the  dis- 
charging rod,  (which  is  a  bent  wire, 
armed  at  both  ends  with  knobs,  and  in- , 
sulated  by  a  glass  handle,  as  in  figure 
61,)  and  apply  one  of  the  knobs  to  the 
outer  coating  and  bring  the  other  to 
the  knob  of  the  jar,  a  flash  of  intense 
brightness,  accompanied  by  a  loud  re- 
port, immediately  ensues.  If,  instead 
of  the  discharging  rod,  we  apply  one 
hand  to  the  outside  of  the  charged  jar, 
and  bring  a  knuckle  of  the  other  hand  to  the  knob  of 
the  jar,  a  sudden  and  surprising  shock  is  felt,  convul- 


Fig.  61. 


138.  Define  the  Leyden  Jar— describe  it — how  is   it  charged  1 
[ow  discharged  1    How  is  the  shock  taken  1 


136  NATURAL  PHILOSOPHY. 

sing  the  arms,  and  when  sufficiently  powerful,  passing 
through  the  breast. 

139.  The  outside  and  the  inside  of  a  Leyden  Jar  are 
always  found  in  opposite  states  ;  that  is,  if  to  the  knob  I 
connected  with  the  inside  we  have  imparted  positive'; 
electricity,  (as  in  the  mode  of  charging  already  de-| 
scribed,)  then  the  outside  will  be  electrified  in  the  same 
degree  with  negative  or  resinous  electricity.  Every 
spark  of  one  sort  of  fluid  that  enters  into  the  jar,  drives 
off  a  spark  of  the  same  kind  from  the  outside,  and 
leaves  that  in  the  opposite  state.  And  if  the  jar  is  in- 
sulated, (as  when  it  stands  on  a  glass  support,)  so  that 
the  electricity  cannot  pass  from  the  outer  coating,  then 
it  will  take  no  charge.  We  may  charge  a  jar  nega- 
tively instead  of  positively,  'by  grasping  hold  of  the 
knob  and  presenting  the  outside  to  the  prime  conductor. 
The  positive  electricity  that  enters  the  outer  coating, 
drives  off  an  equal  quantity  of  the  same  kind  from  the 
inside,  which  escapes  through  the  body  of  the  operator 
and  leaves  the  inner  coating  negative.  When  the  jar 
is  thus  charged,  we  must  be  careful  to  set  it  down  on 
a.- glass  support  before  withdrawing  the  hand;  for  if 
we  place  it  on  the  table,  which  is  a  conductor,  the  elec- 
tricity will  immediately  rush  from  the  outside  to  the 
inside,  through  the  table,  floor,  and  body  of  the  opera- 
tor, and  he  will  receive  a  shock.  But  if  he  sets  the 
jar  on  a  non-conducting  support,  no  such  communica- 
tion will  be  formed  between  the  two  sides  of  the  jar, 
and  consequently  it  will  not  discharge  itself. 

The  Electrical  Spider  forms  a  pleasing  illustration 
of  the  different  states  of  two  jars,  one  charged  posi- 
tively and  the  other  negatively.  It  is  contrived  as  fol- 
lows :  Take  a  bit  of  cork  and  form  a  small  ball  of  the 
size  of  a  pea,  for  the  body  of  the  spider.  With  a 
needle,  pass  a  fine  black  thread  backward  and  for- 

139.  In  what  state  are  the  two  sides  of  a  charged  jar  1  How  may 
we  charge  a  jar  negatively  1  Why  is  it  necessary  to  set  it  down  on  an 


ELECTRICITY. 


137 


Fig.  62. 


ward  through  the  sides  of  the  cork,  letting  the  threads 
project  from  it  half  or  three  fourths  of  an  inch  on  the 
opposite  sides,  to  form  the 
legs.  Now  suspend  it  from 
the  center  of  the  body  by 
a  fine  silk  thread,  between 
two  jars,  one  charged  posi- 
tively and  the  other  nega- 
tively, and  placed  on  a  table, 
as  is  represented  in  figure 
62.  The  spider  will  first 
be  attracted  to  the  knob  of 
the  nearest  jar,  will  imbibe 
the  same  electricity,  be  re- 
pelled, and  attracted  to  the 
knob  of  the  other  jar,  from 
which  again  it  will  be  repelled,  and  so  will  continue 
to  vibrate  back  and  forth  between  the  two  jars,  until  it 
has  restored  the  equilibrium  between  them  by  slowly 
conveying  to  the  inside  of  each  jar  the  electricity  of 
the  inner  coating  of  the  other. 

Pointed  conductors  have  a  remark- 
able powrer  of  drawing  off  and  dis- 
sipating the  electric  fluid  when  it 
has  accumulated.  If  we  apply 
one  hand  to  the  outer  coating  of  a 
charged  jar,  and  with  the  other  bring 
a  needle  towards  the  knob,  it  will 
silently  draw  off  all  the  charge, 
without  giving  any  shock.  And  if," 

j  while  we  are  charging  a  jar  with  the 

1  machine,  we  direct  a  pointed  wire 
or  a  needle  towards  the  machine,  even  at  a  much  greater 
distance  from  it  than  the  knob  of  the  jar,  the  fluid  will 

insulated  support  1    Describe  the  electrical  spider.    Why  does  it 
vibrate  from  one  jar  to  the  other  1    Effect  of  points. 


Fig.  63. 


138  NATURAL   PHILOSOPHY. 

pass  into  the  needle  in  preference  to  the  jar.  All 
apparatus,  therefore,  for  confining  electricity,  requires 
to  be  free  from  sharp  lines  and  points,  and  to  terminate 
in  round  smooth  surfaces. 

SEC.  3.  Of  Electrical  Light  and  Heat. 

140.  Electrical  Light  appears  whenever  the  fluid  is 
discharged  in  considerable  quantities  through  a  resist- 
ing medium.  When  electricity  flows  freely  through 
good  conductors,  it  exhibits  neither  light  nor  heat  ;  but 
if  such  conductors  suffer  any  interruption,  as  in  pass- 
ing through  a  small  space  of  air,  or  even  through  an 
imperfect  conductor,  then  light  becomes  manifest.  We 
will  suppose  the  experiment  to  be  performed  in  a  dark 
room,  or  in  the  evening,  in  a  room  very  feebly  lighted. 
A  glass  tube,  rubbed  with  black  silk,  coated  with  a 
little  electrical  amalgam,  will  afford  numerous  sparks, 
with  a  slight  crackling  noise.  A  chain,  hung  to  the 
prime  conductor  of  a  machine,  will  show  a  bright 
spark  at  every  link.  If  we  attach  one  end  of  the 
chain  to  the  prime  conductor,  and  hold  the  other 
end  suspended  by  a  glass  tube,  brushes  or  pencils 
of  light  will  issue  from  various  points  along  the 
chain.  The  spark  seen  in  discharging  the  Leyden 
Jar,  as  in  Article  138,  is  very  intense  and  dazzling. 
Fig.  64. 


Figure  64  represents  a  glass  cylinder,  armed  at  each 
end  with  brass  balls,  and  wound  round,  spirally,  with 
a  narrow  strip  of  tin  foil.  At  short  intervals,  small 
portions  of  the  tin  foil  are  cut  out,  so  as  to  interrupt 

140.  When  does  electrical  light  appear  1  When  does  electricity 
exhibit  neither  light  nor  heat  1  Experiment  with  a  glass  tube  —  a 
chain—  a  spiral  tube  1  How  may  illuminated  words  be  made  to 
appear  1 


ELECTRICITY.  139 

the  circuit.  Whenever  a  spark  is  passed  through  this 
apparatus,  it  appears  beautifully  luminous  at  every 
interruption  in  the  tin  foil.  Words  or  figures  of  any 
kind  may  be  very  finely  exhibited  by  coating  a  plate 
of  glass  with  a  strip  of  tin  foil  in  a  zigzag  line,  from 
one  corner  to  the  opposite  corner,  diagonally.  Then 
with  the  point  of  a  knife,  small  portions  of  the  tin  foil 
are  nicked  out  in  such  a  manner  that  the  spaces 
thus  left  bare  shall  together  constitute  some  word,  as 
WASHINGTON.  The  spark,  in  passing  through  the  tin 
foil,  will  meet  with  resistance  at  all  the  places  where 
the  metal  has  been  removed,  and  will  there  exhibit  a 
bright  light.  Thus  an  illuminated  word  will  appear 
at  every  spark  received  from  the  machine.  If  the 
machine  is  not  sufficiently  powerful  to  afford  a  spark 
strong  enough  to  overcome  the  resistance  occasioned 
by  so  many  non-conducting  spaces,  then  the  illu- 
minated word  may  be  made  to  appear  with  great 
splendor,  by  making  the  plate  form  a  part  of  the  cir- 
cuit between  the  inside  and  the  outside  of  a  charged 
Leyden  Jar. 

141.  By  means  of  the  Battery,  far  more  brilliant 
experiments  may  be  performed  than  with  a  single  jar. 
The  Battery  consists  of  a  number  of  jars,  twelve,  for 
instance,  so  combined  that  the  whole  may  be  either 
charged  or  discharged  at  once.  Large  Leyden  Jars, 
placed  side  by  side  in  a  box,  standing  on  tin  foil,  which 
forms  a  conducting  communication  between  the  outer 
coatings,  while  the  inner  coatings  are  also  in  commu- 
nication by  a  system  of  wires  and  knobs,  answer  the 
same  purpose  as  a  single  jar  of  enormous  size,  and  are 
far  more  convenient.  When  the  battery  is  charged, 
and  a  chain  is  made  to  form  a  part  of  the  circuit 
between  the  outside  and  inside,  on  discharging  it,  the 
whole  chain  is  most  brilliantly  illuminated.  Rough 

141.  The  Battery— of  what  does  it  consist  1  Describe  it.  How  is 
a  chain  illuminated  by  the  battery  1  Great  power  of  some  batteries. 


140  NATURAL   PHILOSOPHY. 

lightning  rods  sometimes  present  a  similar  appearance 
when  struck  during  a  thunder  storm.  Batteries  are 
sometimes  made  of  sufficient  power  to  kill  small  animals, 
and  even  men. 

142.  Heat,  as  well  as  light,  attends  the  electric  spark, 
although,  except  when  the  discharge  is  very  powerful, 
as  in  the  case  of  the  battery,  or  of  lightning,  it  is  but 
feeble,  sufficient  to  set  on  fire  only  the  most  inflammable 
substances.     Alcohol  and  ether,  two  very  inflammable 
liquids,  may  be  fired  by  the  spark,  a  candle  may  be 
lighted,   and   gunpowder  exploded.      It   is,    however, 
difficult  to  set  powder  on  fire  by  electricity,  unless  the 
spark  is  very  strong. 

143.  The  electric  spark  passes  much  more  easily 
through  rarefied  air,  than  through  air  in  its  ordinary 
state.     Thus,  a  spark  which  would  not  strike  through 
the  air  more  than  four  or  five  inches,  will  pass  through 
an  exhausted  glass  tube,  four  feet  or  more  in  length, 
filling  all  the  interior  with  a  soft  and  flickering  light, 
somewhat  resembling  the  Aurora  Borealis.  •  Hence,  that 
phenomenon  has  been  ascribed  by  some  to  electricity, 
though  this  is  probably  not  its  true  explanation. 

144.  In  Thunder  Storms,  we  see  electricity  exhibited 
in  a  state  of  accumulation  far  beyond  what  we  can 
create  by  our  machines,  and  producing  effects  propor- 
tionally more  energetic.    A  cloud  presents  a  conductor 
insulated   by  the  surrounding  air,  in  which,  in  hot 
weather,  electricity  collects    and   accumulates  as    it 
would  upon  a  prime  conductor  of  immense  size.     By 
sending  up  a  kite  armed  with  points,  electricity  may 
be  drawn  from  such  clouds,  and  made  to  descend  by 
a  wire  wound  round   the   string  of   the  kite.      We 

142.  Does  heat  attend  electricity  1    Give  examples  of  bodies  fired 
by  it. 

143.  How  does  the  spark  pass  through  rarefied  air  1    Explain  the 
appearance  of  the  Auroral  tube. 

144.  How  is  electricity  exhibited  in  thunder  storms  1    Analogy  be- 
tween a  cloud  and  a  prime  conductor.   How  may  lightning  be  drawn 


ELECTRICITY.  141 

may  easily  direct  it  upon  a  prime  conductor,  or  charge 
a  Leyden  Jar  with  it,  and  examine  its  properties  as  we 
should  do  in  the  case  of  ordinary  electricity.  By  such 
experiments,  it  is  found  that  the  clouds  are  sometimes 
positively  and  sometimes  negatively  electrified.  In 
thunder  storms,  the  lightning  is  usually  nothing  more 
than  the  electric  spark  passing  from  one  cloud  to  an- 
other differently  electrified,  as  it  passes  between  the 
outer  and  inner  coating  of  the  Leyden  Jar.  The  flash 
appears  in  the  form  of  a  line,  because  it  passes  so 
swiftly,  just  as  a  stick,  lighted  at  the  end  and  whirled 
in  the  air,  forms  a  circle  of  light.  The  motion  of  the 
electric  fluid  is,  to  all  appearance,  instantaneous. 
Thunder  is  the  report  occasioned  by  the  rushing  to- 
gether of  the  air,  after  it  has  been  divided  by  the  pas- 
sage of  the  lightning.  The  cracking*  of  a  whip,  as  al- 
ready mentioned,  is  ascribed  to  the  same  cause.  The 
lash  divides  the  air  into  two  parts,  which  forcibly  rush 
together  and  occasion  the  sound.  When  a  thunder- 
clap is  very  near  us,  the  report  follows  the  flash  almost 
instantly,  and  such  claps  are  dangerous.  In  all  cases, 
the  lightning  and  the  thunder  actually  occur  at  the 
same  moment,  but  when  the  discharge  is  at  some  dis- 
tance from  us,  the  report  is  not  heard  till  some  time 
after  the  flash  ;  for  the  light  reaches  the  eye  instanta- 
neously, but  the  sound  travels  with  comparative  slow- 
ness, moving  only  about  a  mile  in  five  seconds.  We 
may,  therefore,  always  know  nearly  how  distant  a 
thunder  cloud  is,  by  counting  the  number  of  seconds 
between  the  flash  and  the  report,  and  allowing  the  fifth 
of  a  mile  (or,  more  accurately,  1,130  feet)  to  a  second. 
(See  Art.  124.) 

145.  Sometimes  lightning,  instead  of  passing  from 

from  the  clouds  *?  How  is  the  flash  produced  in  thunder  storms  1 
Why  does  it  leave  a  bright  line  1  What  is  thunder  1  How  produced  1 
Why  are  the  flash  and  the  report  sometimes  together  and  sometimes 
separate  1 


142  NATURAL   PHILOSOPHY. 

cloud  to  cloud,   discharges  itself  into  the  earth,  and 
then  strikes  objects  that  come  in  its  route,  as  houses, 
trees,  animals,   and  sometimes  man.     As   electricity 
always  selects,  in  its  passage,  the  best  conductors,  Dri 
Franklin   first  suggested   the   idea  of  protecting   our 
dwellings  by  means  of  Lightning  Rods.     If  these  are: 
properly  constructed,  the  lightning  will  always  take  its  < 
passage  through  them  in  preference  to  any  part  of  the 
house,  and  thus  they  will  afford  complete  protection  to  • 
the  family.     Sharp  metallic  points  were  observed  by 
Dr.  Franklin  to  have  great  power  to  discharge  elee-1 
tricity  from  either  a  prime  conductor  or  a  Leyden  Jar, 
and  this  suggested  their  use  in  lightning-rods.     Metals,  s 
also,  being  the  best  conductors  of  electricity,    would 
obviously  afford  the  most  proper  material  for  the  bodyj 
of  the  rod. 

There  are  three  or  four  conditions  in  the  construc- 
tion and  application  of  a  lightning-rod,  which  are  es-  ' 
sential  to  insure  complete  protection.     The  rod  must  1 
not  be  less  than  three-fourths  of  an  inch  in  diameter — 
it  must  be  continuous  throughout,  and  not  interrupted 
by  loose  joints — it  must  terminate  above  in  one  or  more 
sharp  points  of  some  metal,  as  silver,  gold,  or  platina,  ] 
not  liable  to  rust — it  must  enter  the  ground  to  the  depth 
of  permanent  moisture,  which  will  be  different  in  dif-J 
ferent  soils,  but  usually  not  less  than  six  feet.     A  rod 
thus  constructed  will  generally  protect  a  space  every " 
Way  equal  to  twice  its  height  above  the  ridge  of  the 
house.     Thus,  if  it  rises  fifteen  feet  above  the  ridge,  it 
will  protect  a  space  every  way  from  it  of  thirty  feet. 
It  is  usually  best  to  apply  the  rod  to  the  chimney  of 
the  house  •  or,  if  there  are  several  chimneys,  it  is  best 
to   select   one   as    central  as  possible.     The   kitchen 

145.  What  happens  when  lightning  strikes  to  the  earth  1  Lightning- 
rods — influence  of  points  and  conductors — power  of  metals — size  of 
the  rod — to  be  continuous — how  terminated  above  and  below  1  How 
much  space  will  a  rod  protect  1  How  applied  to  a  house  1  What  is 


ELECTRICITY.  143 

chimney,  being  usually  the  only  one  in  which  fires  are 
maintained  during  the  season  of  thunder  storms,  re- 
quires to  be  specially  protected,  since  a  column  of 
smoke  rising  from  a  chimney  is  apt  to  determine  the 
course  of  the  lightning  in  that  direction.  If,  therefore, 
the  lightning-rod  is  attached  to  some  other  chimney  of 
the  house,  either  a  branch  should  proceed  from  it  up 
the  kitchen  chimney,  or  this  should  have  a  separate 
rod.  As  lightning,  in  its  passage  from  a  cloud  to  the 
-earth,  selects  tall  pointed  objects,  it  often  strikes  trees, 
and  it  is,  therefore,  never  safe  to  take  shelter  under  trees 
during  a  thunder  storm.  Persons  struck  down  by  light- 
ning are  sometimes  recovered  by  dashing  on  repeated 
buckets  of  water. 

SEC.  4.  Of  the  Effects  of  Electricity  on  Animals. 

146.    When    we    apply    a  Fig.  65. 

knuckle  to  the  prime  conduct- 
or of    an    electrical  machine, 
and  receive  the  spark,  a  sharp 
and   somewhat   painful  sensa- 
tion is  felt.     If  we  receive  the- 
charge   of    a   Leyden   Jar,    a 
shock  is  experienced  which  is 
more  or    less    severe,    accord-, 
ing  to  the  size  and  power  of  J 
the    jar.     A    battery   gives  a 
shock   still  more    severe,    and 
it    may   be    even    dangerous. 
Lightning,    it  is  well    known, 
sometimes  prostrates  and  kills  men  and  animals.     A 
convenient  method  of  taking  the  shock,  is  to  charge  a 

said  of  the  kitchen  chimney  *?    May  we  take  shelter  under  trees  1 
How  to^  restore  people  struck  by  lightning  1 

146.  Sensation  to  the  knuckle — effects  of  a  jar — of  a  battery.  What 
is  a  convenient  mode  of  .taking  the  shock  1   Sensations  produced  by 


144 


NATURAL   PHILOSOPHY. 


quart  jar,  place  it  on  a  table,  and  grasping  in  each  hand 
a  metallic  rod,  apply  one  rod  to  the  outside  of  the  jar, 
and  touch  the  other  to  the  knob  connected  with  the  in- 
side. If  the  charge  is  feeble,  it  will  be  felt  only  in  the 
arms  ;  if  it  is  stronger,  it  will  be  felt  in  the  breast ; 
and  it  may  be  sufficiently  powerful  to  convulse  the 
whole  frame.  Any  number  of  persons  may,  by  taking 
hold  of  hands,  all  receive  the  shock  at  the  same  instant. 
The  first  must  touch  the  outside,  and  the  last  the  knob 
of  the  jar.  Whole  regiments  have  been  electrified  at 
once  in  this  way. 

147.  Electricity  is  sometimes  employed  medicinally, 
and  is  thought  to  afford  relief  in  various  diseases.  It 
may  be  applied  either  to  the  whole  system  at  once,  or 
to  any  individual  part,  by  making  that  part  form  a  por- 
tion of  the  communication  between  the  inside  and  the 

outside  of  a  jar.  Or  the 
Fig.  66.  fluid  may  be  taken  in  a 

milder  form  by  means 
of  the  Electrical  Stool. 
This  is  a  small  stool, 
resting  on  glass  feet. 
The  patient  stands  or 
sits  on  the  stool,  and 
holds  a  chain  connect- 
ed with  the  prime  con- 
ductor, while  the  ma- 
chine is  turned.  This 
produces  an  agreeable 
excitement  over  the 
whole  system :  the  hair  stands  on  end ;  sparks  may 
be  taken  from  all  parts  of  the  person,  as  from  a  prime 
conductor ;  and  the  patient  may  communicate  a  slight 

a  feeble  charge — by  a  strong — by  a  powerful  charge  1  How  may 
any  number  of  persons  be  electrified  at  once  1 

147.  How  is  electricity  employed  medicinally!    How  by  means 
of  the  electrical  stool  7 


MAGNETISM.  145 

I  shock  to  any  one  that  comes  near  him,  or  may  set  on 
fire  ether  and  other  inflammable  substances,  by  merely 
touching  them  with  a  rod,  or  pointing  toward  them. 

148.  Several  fishes  have  remarkable  electrical  pow- 
ers. Such  are  the  Torpedo,  the  Gymnotus,  and  the 
Silurus.  The  Gymnotus,  or  Surinam  eel,  is  found  in 
the  rivers  of  South  America.  Its  ordinary  length  is 
from  three  to  four  feet ;  but  it  is  said  to  be  sometimes 
twenty  feet  long,  and  to  give  a  shock  that  is  instantly 
fatal.  Thus,  it  paralyzes  fishes,  which  serve  as  its 
food,  and  in  the  same  manner  it  disables  its  enemies 
and  escapes  from  them.  By  successive  efforts,  elec- 
trical fishes  exhaust  themselves.  In  South  America,  the 
natives  have  a  method  of  taking  them,  by  driving  wild 
horses  into  a  lake  where  they  abound.  Some  of  the 
eels  are  very  large,  and  capable  of  giving  shocks  so 
powerful  as  to  disable  the  horses ;  but  the  eels  them- 
selves are  so  much  exhausted  by  the  process,  as  to  be 
easily  taken. 


CHAPTER  VIII. 
MAGNETISM. 

DEFINITIONS ATTRACTIVE      PROPERTIES DIRECTIVE      PROPERTIES 

— VARIATION  OF  THE  NEEDLE DIP MODES  OF  MAKING  MAGNETS. 

149.  AMONG  the  ores  of  iron,  there  is  found  an  ore 
of  a  peculiar  kind,  which  has  the  power  of  attracting 
iron  filings,  and  other  forms  of  metallic  iron,  and  is 
called  the  loadstone.  This  power  can  be  imparted  to 
bars  of  steel,  which  are  denominated  magnets.  The 
unknown  power  which  produces  the  peculiar  effects 
of  the  magnet,  is  called  magnetism.  This  name  is 

148.  What  of  electrical  rishes  1    Give  an  account  of  the  Gymno- 
tus.    How  do^the  natives  take  electrical  fishes  in  South  America'? 

149,  What  is  the  loadstone,  and  magnets  1    Define  Magnetism — 

13 


146 


NATURAL   PHILOSOPHY. 


also  applied,  as  at  the  head  of  this  chapter,  to  that 
branch  of  Natural  Philosophy  which  treats  of  the 
magnet.  Magnetic  bars  are  thick  plates  of  iron  or 
steel,  commonly  about  six  inches  long.  If  a  magnetic 
bar  be  placed  among  iron  filings,  they  will  arrange 
themselves  around  a  point  at  each  end,  forming  tufts, 

Fig.  67. 


as  is  shown  in  figure  67.  These  two  points  are  called 
the  poles,  and  the  straight  line  that  joins  them,  the 
axis  of  the  magnet.  If  we  suspend,  by  a  fine  thread, 
a  small  needle,  and  approach  toward  it  either  poles 
of  a  metallic^  bar,  the  needle  will  rush  toward  itj 
and  attach  itself  strongly  to  the  pole.  By  rubbing 
the  needle  on  one  of  the  poles  of  the  magnet  itl 
will  itself  imbibe  the  same  power  of  attracting  iron, 
and  become  a  magnet,  having  its  poles.  If  we  now 
bring  first  one  pole  of  the  mag- 
Fig.  68.  Fig.  69.  netic  bar  toward  the  needle,  andv 
1  then  the  other  pole,  we  shall  find 
that  one  attracts,  and  the  other  re-^ 
pels  the  needle.  Figure  68  repre- 
sents two  large  sewing  needles, 
magnetized,  and  suspended  by  fine 
threads.  On  approaching  the  north 
pole  of  a  'magnetic  bar  to  the  north 
poles  of  the  needles,  they  are 
forcibly  repelled ;  but  on  apply- 
ing the  south  pole  of  a  bar,  as  in 
figure  69,  the  north  poles  of  the 
edles  are  attracted  toward  it. 

two  senses  in  which  the  word  is  used.  What  are  magnetic  bars  1 
What  are  the  poles— the  axis  1  How  may  a  needle  be  magnetized  1 
How  are  its  properties  changed  by  this  process  1 


MAGNETISM.  147 

150.  Let  us  suppose  that  the  long  needle  represent- 
ed in  figure  70,  has  been  rubbed  on  a  magnet,  so  as  to 
imbibe  its  properties,  or  to 
become  magnetized  ;  then,  on 
balancing  it  on  a  pivot,  it  will  s 
of  its  own  accord  place  itself 
in  nearly  a  north  and  south 
line,  and  return  forcibly  to 
this  position  when  drawn 
aside  from  it.  This  property 
is  called  the  directive,  while  the  other  is  called  the 
attractive,  property  of  the  magnet.  That  end  which 
points  northward,  is  called  the  North  Pole  of  the 
magnet,  and  that  end  which  points  southward,  is 
called  the  South  Pole.  Every  magnet  has  these  two 
poles,  whatever  may  be  its  size  or  shape.  A  mag- 
netic bar  has  usually  a  mark  across  one  end,  to  de- 
note that  it  is  the  north  pole,  the  other,  of  course, 
being  the  south  pole.  If  the  north  pole  of  a  bar  be 
brought  toward  the  north  pole,  N,  (Fig.  70,)  of  the 
needle,  it  will  repel  it,  and  the  more  forcibly  in  pro- 
portion as  we  bring  it  nearer  to  N.  On  the  contrary, 
if  the  north  pole  of  the  bar  be  brought  toward  the 
south  pole  S  of  the  needle,  it  will  •  attract  it.  Also, 
if  we  present  the  south  pole  of  the  bar  first  to  one  pole 
of  the  needle,  and  then  to  the  other,  we  shall  find  that 
the  bar  will  repel  the  pole  of  the  same  name  with  its 
own,  and  attract  its  opposite.  These  facts  are  ex- 
pressed by  the  proposition  that  similar  poles  repel,  and 
opposite  poles  attract  each  other.  When  a  magnetic 
bar  is  laid  on  a  sheet  of  paper,  and  iron  filings  are 


150.  Explain  the  directive  property.  "Which  is  the  north  and 
which  the  south  pole  1  How  is  the  north  pole  distinguished  1  Ef- 
fect when  the  north  pole  of  the  bar  is  brought  near  the  north  pole 
of  the  needle — when  the  north  pole  is  brought  toward  the  south 

Eole  1    State  the  general  fact,     w  hat  takes  place  when  a  magnetic 
ar  is  placed  among  iron  filings  1 


148  NATURAL  PHILOSOPHY. 

sprinkled  on  it,  they  will  arrange  themselves  in  curves 
around  it,  as  in  figure  71. 

Fig.  71. 


151.  The  magnetic  needle,  when  freely  suspended, 
seldom  points  directly  to  the  pole  of  the  earth,  but  its 
deviation  from  that  pole,  either  east  or  west,  is  called 
the  variation  of  the  needle.  A  line  drawn  on  the  sur- 
face of  the  earth,  due  north  and  south,  is  called  a  me- 
ndian  line.  The  needle  usually  makes  a  greater  or| 
less  angle  with  this  line.  Its  direction  is  called  the 
magnetic  meridian,  and  the  place  on  the  earth  to  which 
it  points,  is  called  the  magnetic  pole.  The  earth  has 
two  magnetic  poles,  one  in  the  northern,  the  other  in 
the  southern  hemisphere.  The  north  magnetic  pole 
is  in  the  part  of  North  America  lying  north  of  Hud- 
son's  and  west  of  Baffin's  Bay,  in  latitude  70°.  The 
variation  of  the  needle  is  different  in  different  coun- 
tries. In  Europe,  the  needle  points  nearly  N.  W. 
and  S.  E.  ;  while  in  the  United  States  it  deviates  no-  ' 
where  but  a  few  degrees  from  north  and  south  ;  and 
along  a  certain  series  of  places,  passing  through  West- 
ern New  York  and  Pennsylvania,  the  variation  is  noth- 
ing ;  that  is,  the  needle  points  directly  north  and  south. 
At  the  same  place,  moreover,  the  variation  of  the 
needle  is  different  at  different  periods.  For  a  long 
series  of  years,  the  needle  will  slowly  approach  the 

151.  "What  is  meant  by  the  variation  of  the  needle  1  What  is  a 
meridian  line  1 — the  magnetic  meridian  *?  Situation  of  the  north  mag- 
netic pole  ]  How  is  the  variation  of  the  needle  in  Europe  1  How  in 
the  United  States  1  Where  does  the  line  of  no  variation  run  1  How 


MAGNETISM. 


149 


!  North  pole,  come  within  a  certain  distance  of  it,  and 
then  turn  about  and  again  slowly  recede  from  it.  At 
Yale  College,  the  variation  in  1843,  was  6i  degrees 
West,  and  is  increasing  at  the  rate  of  4J  minutes  a  year. 

152.  A  needle  first  balanced  on  its  center  of  gravity, 
and    then    magnetized,   no 

longer  retains  its  level,  but  Fig.  72. 

it  points  below  the  horizon, 
making  an  angle  with   it, 
called  the  Dip  of  the  needle. 
The    dipping    needle    is 
shown  in  figure  72,  adapted  . 
to  a  graduated  circle  in  or-  r — 
der  to  indicate  the  amount  of  \ 
the  depression,  and  is  some- 
times fitted  with  screws  and 
a  level  to  adjust  it  for  obser- 
vation .    The  dip  of  the  nee- 
dle varies  very  much  in  dif- 
ferent parts  of  the  earth,  being  in  general  least  in  the 
equatorial,  and  greatest  in  the  polar  regions.     At  Yale 
College,  it  is  about  73  degrees,  being  greater  than  is 
exhibited  in  the  figure. 

153.  The  directive  property  of  the  needle  has  two 
most  interesting  and  important  practical  applications, 
in  surveying  and  navigation.     The  compass  needle, 
in  order  to  keep  it  at  a  horizontal  level,  and  prevent  its 
dipping,  has  a  counterpoise  on  one  side,  which  exactly 
balances  the  tendency  to  point  downward.     By  the 
aid  of  this  little  instrument,  lands  are  measured,  and 
boundaries  determined  ;   the  traveller  finds  his  way 

does  the  variation  change  at  any  given  place  1    How  is  it  at  New 
Haven  1 

152.  What  is  the  Dip  of  the  needle  1  Describe  figure  72.    "Where 
is  the  dip  of  the  needle  greatest  1    Where  least  1    Its  amount  at 
Yale  College  1 

153.  What  are  the  two  leading  applications  of  the  needle  1  How  is 
the  compass  needle  kept  from  dipping  !   To  what  uses  is  it  applied  1 

13* 


150  NATURAL  PHILOSOPHY. 

through  unexplored  forests  and  deserts  ;  and  mariners 
guide  their  ships  through  darkness  and  tempests,  and 
across  pathless  oceans. 

154.  There  are  various  methods  of  making  compass 
needles,  or  artificial  magnets.  Soft  iron  readily  receives 
magnetism,  but  as  readily  loses  it ;  hard  steel  receives 
it  more  slowly,  but  retains  it  permanently.  It  is  a 
singular  property  of  a  magnet,  whether  natural  or 
artificial,  that,  like  virtue,  it  loses  nothing  by  what  it 
imparts  to  another.  In  fact,  such  an  exercise  of  its 
powers  is  essential  to  their  preservation.  The  strongest 
magnet,  if  suffered  to  remain  unemployed,  gradually 
loses  power.  Magnets,  therefore,  and,  loadstones,  are 
kept  loaded  with  as  much  iron  as  they  are  capable  of 
holding,  called  their  armature.  If  we  simply  rub  a 
penknife  on  one  pole  of  a  magnet,  we  render  it  magnetic, 
as  will  be  indicated  by  its  taking  up  iron  filings  or 
sewing  needles.  Magnetism  is  most  readily  imparted 
by  a  bar,  when  both  its  poles  are  made  to  act  together. 
This  is  done  by  giving  the  bar  the  form  of  a  horse-shoe, 
as  in  figure  73.  To  magnetize  a  needle,  we  lay  it  flat 

on  a  table,  and  place  the 
Fig.  73.  two  poles  of  the  horse- 

shoe  magnet  near  the 
middle,  and  rub  it  on  the 
needle,  backward  and 
forward,  first  toward  one 
end  and  then  toward  the 

•other,  taking  care  to  pass  over  each  half  of  it  an  equal 
number  of  times.  The  needle  may  then  be  turned  over, 
and  the  same  process  performed  on  the  other  side,  when 
it  will  be  found  strongly  and  permanently  magnetized. 

154.  How  is  the  compass  needle  made  1  "What  is  said  of  soft  iron 
and  hard  steel  1  How  is  the  strength  of  the  magnet  affected  by 
action  or  inaction  1  What  is  the  armature  1  How  to  magnetize  a 
penknife.  Why  is  a  bar  bent  into  the  horse-shoe  form  1  How  to 
magnetize  a  needle  with  it. 


CHAPTER  IX. 
OPTICS. 

DEFINITIONS — REFLEXION  AND    REFRACTION — COLORS VISION — MI- 
CROSCOPES  AND    TELESCOPES. 

155.  OPTICS  is  that  branch  of  Natural  Philosophy 
which  treats  of  Light.  Light  proceeds  from  the  sun,  a 
lamp,  and  all  other  luminous  bodies,  in  every  direction, 
in  straight  lines,  called  rays.  If  it  consists  of  matter, 
its  particles  are  so  small  as  to  be  incapable  of  being 
weighed  or  measured,  many  millions  being  required 
to  make  a  single  grain.  Some  bodies,  as  air  and  glass, 
readily  permit  light  to  pass  through  them,  and  are 
called  transparent ;  others,  as  plates  of  metal,  do  not 
permit  us  to  see  through  them,  and  are  called  opake. 
Any  substance  through  which  light  passes,  is  called 
a  Medium.  Light  moves  with  the  astonishing  velocity 
of  192,500  miles  in  a  second.  It  woulcj  cross  the 
Atlantic  Ocean  in  the  sixty-fourth  part  of  a  second, 
and  in  the  eighth  part  of  a  second,  would  go  round  the 
earth.  When  light  strikes  upon  bodies,  some  portion 
of  it  enters  the  body,  or  is  absorbed,  and  more  or  less 
of  it  is  thrown  back,  and  is  said  to  be  reflected  ;  when 
it  passes  through  transparent  bodies,  it  is  turned  out 
of  its  direct  course,  and  is  said  to  be  refracted.  The 
light  of  the  sun  consists  of  seven  different  colored 
rays,  which,  being  variously  absorbed  and  reflected 
by  different  bodies,  constitute  all  the  varieties  of  colors. 
Light  enters  the  eye,  and  forming  within  it  pictures 
of  external  objects,  thus  gives  the  sensation  of  vision. 
The  knowledge  of  the  properties  of  light,  and  the 
nature  of  vision,  has  given  rise  to  the  invention  of 
many  noble  and  excellent  instruments,  which  afford 

155.  Define  Optics— terms  rays,  transparent,  opake,  and  medium. 
When  is  light  said  to  be  reflected  1  When  refracted  1  Of  what  do  the 


152  NATURAL  PHILOSOPHY. 

wonderful  aid  to  the  eye,  such  as  the  microscope  and 
the  telescope.  Let  us  examine  more  particularly  these 
interesting  and  important  subjects,  under  separate 
heads. 

SEC.  1.  Of  the  Reflexion  and  Refraction  of  Light. 

156.  When  rays  of  light,  on  striking  upon  some  body, 
are  turned  back  into  the  same  medium,  they  are  said 
to  be  reflected.     Smooth  polished  surfaces,  like  mirrors 
and  wares  of  metal,  reflect  light  most  freely  of  any,  and 
hence  their  brightness.      Most  objects,  however,  are 
seen  by  reflected  light ;  few  shine  by  their  own  light. 
Thus,  the  whole  face  of  nature  owes  its  brightness  and 
its  various  colors  to  the  light  of  the  sun  by  day,  and  \ 
to  the  light  of  the  moon  and  stars  by  night.     The  rays 
that  come  from  these  distant  luminaries,  fall  first  upon 
the  atmosphere,  and  are  so  reflected  and  refracted  from 
that  as  to  light  up  the  whole  sky,  which,  were  it  not  for 
such  a  power  of  scattering  the  rays  of  light  that  fall 
upon  it,  would  be  perfectly  black.     On  account  of  the^ 
transparency  of  the  atmosphere,  the  greater  part  of 
the  sun's  rays  pass  through  it,  and  fall  upon  the  surface 
of  the  earth,  and  upon  all  objects  near  it.    These  reflect 
the  light  in  various  directions,  and  are  thus  rendered 
visible  by  that  portion  of  the  light  which  proceeds  from 
them  to  the  eye. 

157.  When  a  ray  of  light  strikes  upon  a  plane 
surface,  the  angle  which  it  makes  with  a  perpendicular 
to  that  surface,  is  called  the  angle  of  incidence,  and  the 
angle  which  it  makes  with  the  same  perpendicular, 
when  reflected,  is  called  the  angle  of  reflexion.     The 

sun's  rays  consist  1  To  what  inventions  has  the  study  of  Optics 
given  rise  *? 

156.  When  are  rays  of  light  said  to  be  reflected  *?    By  what  light 
are  most  objects  seen  1    Show  how  the  atmosphere  and  most  things 
on  the  earth  are  illuminated. 

157.  Define  the  angle  of  incidence  and  of  reflexion.    Equality  be- 


OPTICS. 


153 


angle  of  reflexion  is  equal  to  the  angle  of  incidence. 
Thus,  a  ray  of  light,  A  C,  striking  upon  a  plane  mir- 
ror, M  N,  at  C,  will  be 
reflected  off  into  the  line 
C  B,  making  the  angle  of  A 
incidence,  MCA,  equal N 
to  the  angle  of  reflexion, 
N  C  D.  It  is  not  neces- 
sary that  the  surface  on 
which  the  light  strikes/ 
should  be  a  continuedl 
plane  ;  the  small  part  of M 
a  curved  surface,  on  which 
a  ray  of  light  falls,  may  be  considered  as  a  plane, 
touching  the  curve  at  that  point,  so  that  the  same  law 
of  reflexion  holds  in  curved  as  in  plane  surfaces. 
Now  the  grains  of  sand  on  a  sandy  plain,  present  sur- 
faces variously  inclined  to  each  other,  which  scatter 
the  rays  of  the  sun  in  different  directions,  many  of 
which  enter  the  eye,  and  make  such  a  region  appear 
very  bright ;  while  a  smooth  surface,  like  a  mirror,  or 
a  calm  sheet  of  water,  reflects  the  light  that  falls  on  it 
chiefly  in  one  direction,  and  hence  appears  bright  only 
when  the  eye  is  so  situated  as  to  receive  the  reflected 
beam.  Thus,  the  ocean  appears  much  darker  than 
the  land,  except  when  the  sun  shines  upon  it  at  such 
an  angle  as  to  throw  the  reflected  beam  directly  to- 
ward the  eye,  as  at  a  certain  hour  of  the  morning  or 
evening,  and  then  the  brightness  is  excessive. 

158.  An  object  always  appears  in  the  direction  in 
which  the  last  ray  of  light  from  it  comes  to  the  eye. 
Thus,  we  see  the  sun  below  the  surface  of  a  smooth 
lake  or  river,  because  every  ray  of  light,  being  reflect- 
ed from  the  water  as  from  a  mirror,  comes  to  the  eye 

tween  these  two  angles.  Explain  figure  74.  Does  the  same  law 
hold  for  curved  surfaces  1  Which  appears  darkest,  the  ocean  or 
land,  in  the  light  of  the  sun  1 


154 


NATURAL   PHILOSOPHY. 


in  the  direction  in  which  the  image  appears  ;  and  if 
the  light  of  a  star  were  to  change  its  direction  a  hun- 
dred times  in  coming  through  the  atmosphere,  we  should? 
see  the  star  in  the  direction  of  the  last  ray,  in  the  same 
manner  as  if  none  of  the  other  directions  had  existed. 
This  principle  explains  various  appearances  presented 
by  mirrors,  of  which  there  are  three  kinds — plane, 
concave,  and  convex. 

159.  A  common  looking-glass  furnishes  an  example 
of  a  Plane  Mirror.  If  we  place  a  lamp  before  it,  rays 
of  light  are  thrown  from  the  lamp  upon  every  part  of 
the  mirror,  but  we  see  the  lamp  by  means  of  those  few 
of  the  rays  only  which  are  reflected  to  the  eye  ;  all 
the  rest  are  scattered  in  various  quarters,  and  do  not 
contribute  at  all  to  render  the  object  visible  to  a  spec- 
tator at  any  one  point,  although  they  would  produce, 
in  like  manner,  a  separate  image  of  the  lamp  wherever 
they  entered  an  eye  so  situated  as  to  receive  them. 
Hence,  were  there  a  hundred  people  in  the  room,  each 
would  see  a  separate  image,  and  each  in  the  direction 
in  which  the  rays  came  to  his  own  eye.  We  will  sup- 


Fig.  75. 


pose  M  N  to  be  the  looking-glass,  having  a  harp  placed 

158.  In  what  direction  does  an  object  always  appear  1    Example 
in  the  sun— in  a  star.    What  are  the  three  kinds  of  mirrors  1 

159.  Explain  how  the  image  is  formed  in  a  plane  mirror.    What 
rays  only  enable  us  to  see  the  image  1    Explain  Fig.  75.    How  far 


OPTICS.  155 

before  it,  and  the  eye  of  thev  spectator  at  D.  Of  all 
the  rays  that  strike  on  the  glass,  the  spectator  will  see 
the  image  by  those  only  which  strike  the  mirror  in 
such  a  direction,  A  B,  that  when  reflected  from  the 
mirror  at  the  same  angle  on  the  other  side,  they  shall 
enter  the  eye  in  the  direction  B  D.  The  image  will 
appear  at  C,  and  will  be  just  as  far  behind  the  mirror  as 
the  harp  is  before  it.  This  last  principle  is  an  import- 
ant one,  and  it  must  always  be  remembered,  that  every 
point  in  an  object  placed  before  a  plane  mirror,  will 
appear  in  the  image  just  as  far  behind  the  mirror  as 
that  point  of  the  object  is  before  it ;  so  that  the  image 
will  be  an  exact  copy  of  the  object,  and  just  as  much 
inclined  to  the  mirror.  We  learn,  also,  the  reason 
why  objects  appear  inverted  when  we  see  them  re- 
flected from  water,  as  the  surface  of  a  river  or  lake, 
since  the  parts  of  the  object  most  distant  from  the  wa- 
ter, that  is,  the  top  of  the  object,  will  form  the  lowest 
part  of  the  image. 

160.  If  we  take  a  looking-glass  and  throw  an  image 
of  the  sun  on  a  wall,  on  turning  the  mirror  round  we 
shall  find  that  the  image  moves  over  twice  as  many 
degrees  as  the  mirror  does.  If  the  image  is  at  first 
thrown  against  the  wall  of  a  room,  horizontally,  (in 
which  case  the  mirror  itself  would  be  perpendicular  to 
the  horizon,)  by  turning  the  mirror  through  half  a 
right  angle,  the  place  of  the  image  would  be  changed 
a  whole  right  angle,  so  as  to  fall  on  the  ceiling  over 
head.  A  common  table-glass,  which  turns  on  two 
pivots,  being  placed  before  a  window  when  the  sun  is 
low,  will  furnish  a  convenient  means  of  verifying  this 
principle. 


is  the  image  behind  the  mirror  1  How  far  is  each  point  in  the  im- 
age behind  the  mirror  1  Why  do  objects  appear  inverted  when  re- 
flected from  water  1 

160.  If  a  mirror  be  turned,  how  much  faster  does  the  image  move 
than  the  mirror  1    State  how  the  experiment  is  performed. 


156 


NATURAL  PHILOSOPHY. 


161.  A  Concave  Mirnor  collects  rays  of  light.     If 
we  hold  a  small  concave  shaving-glass,  for  instance, 
toward  the  sun,  it  will  collect  the  whole  beam  of  light 
that  falls  upon  it  into  one  point,   called  the  focus. 
.  Figure  76  will  give  some  idea  oft 
Fig.  76.  the  manner  in  which  parallel  rays 

strike  a  concave  mirror,  converge 
^  to  a  focus,  and  then  diverge.  The 
^  angle  of  reflexion  is  equal  to  the 
j  angle  of  incidence  here,  as  well 
^1  as  in  a  plane  mirror ;  but  the 
perpendicular  to  a  curved  surface 
is  the  radius  of  the  circle  of  which  the  curve  is  a 
part.  Thus,  the  line  C  B  is  the  radius  of  the  con- 


Fig.  77. 


cave  mirror,  M  N,  and,  in  a  circle,  every  radius  is 
perpendicular  to  the  surface.  The  sun's  rays  are 
parallel  to  each  other,  or  so  nearly  so,  that  they  may 
be  considered  as  parallel ;  and  when  rays  fall  upon 
the  mirror,  in  the  lines  A  B  and  E  G,  they  are  re- 
flected on  the  other  side  of  the  perpendiculars,  meet- 
ing in  a  common  focus,  F,  which  point  is  called  the 
focus  of  parallel  rays.  Into  this  point,  or  a  small 
space  around  it,  a  concave  mirror  will  collect  a  beam 

161.  What  is  the  office  of  a  concave  mirror  1  Experiment  with  a 
shaving-glass.  What  forms  the  perpendicular  to  a  concave  surface  T 
What  is  the  point  called  where  parallel  rays  are  collected  1  What  \a 


OPTICS.  157 

; of  the  sun,  increasing  in  heat  in  the  same  proportion 
| as  the  illuminated  space  at  F  is  less  than  the  whole 
surface  of  the  mirror.  In  large  concave  mirrors, 
the  heat  at  the  focus  often  becomes  very  powerful, 
iso  as  not  only  to  set  combustibles  on  fire,  but  even 
jto  melt  the  most  infusible  substances.  Hence  the 
iname  focus,  which  means  a  burning  point.  If  a  lamp 
iis  placed  at  F,  the  rays  of  light  proceeding  from  it  in 
the  lines  F  G  and  F  B,  will  strike  upon  the  mirror 
and  be  reflected  back  into  the  parallels,  G  E  and  B  A. 
We  shall  see  hereafter  how  useful  this  property  of 
;concave  mirrors, — to  collect  parallel  rays  of  light  into 
a  focus, — is  in  the  construction  of  that  most  noble  of 
instruments,  the  telescope. 

A  Convex  Mirror,  on  the  other  ha^id,  separates  rays 
lof  light  from  each  other,  still  observing  the  same  law, 
that  of  making  the  angle  of  incidence  equal  to  the 


/ingle  of  reflexion.  In  figure  78,  the  parallel  rays, 
jA  B,  0  D,  E  F,  are  represented  as  falling  on  a  convex 
mirror,  ^M  N.  A  B  and  E  F,  being  reflected  to  the 
other  sides  of  the  radii,  C  B  and  C  F,  are  separated 


Ssaid  of  the  heat  at  the  focus  1    When  a  lamp  is  placed  in  the  focus, 
now  will  its  light  be  reflected? 

14 


158 


NATURAL   PHILOSOPHY. 


from  each  other,  and  form  the  image  at  I,  which  is 
called  the  imaginary  focus  of  parallel  rays,  because,  at 
this  point,  the  parallel  rays  that  fall  upon  the  mirror 
seem  to  meet  in  a  focus  behind  the  mirror,  and  to 
diverge  again  into  the  lines  B  G  and  F  H. 

162.  Whenever  the  rays  of  light  from  the  different 
parts  of  an  object  cross  each  other  before  forming  the 
image,  the  image  will  be  inverted.  It  is  manifest  from 
figure  79,  that  the  light  by  which  the'  top  of  the  object 


is  represented  forms  the  bottom  of  the  image,  and  the 
light  from  the  bottom  of  the  object  forms  the  top  of  the 
image,  the  two  sets  of  rays  crossing  each  other  at  the 
hole  in  the  screen.  It  is  always  essential  to  the  dis- 
tinctness of  an  image,  that  the  rays  which  proceed 
from  every  point  in  the  object,  should  be  arranged  in 
corresponding  points  in  the  image,  and  should  be  un- 
accompanied by  light  from  any  other  source.  Now  a 
screen  like  that  in  the  figure,  when  interposed,  per- 
mits only  those  rays  from  any  point  in  the  object  that 
are  very  near  together  and  nearly  parallel  to  each  other, 
to  pass  through  the  opening,  after  which  they  continue 
straight  forward  and  form  the  corresponding  point  of 
the  image ;  while  rays  coming  from  any  other  point  in 
the  object  cannot  fall  upon  the  point  occupied  by  the 

162.  In  what  case  will  the  image  appear  inverted  1  Explain  from 
Fig.  79.  What  is  essential  to  the  distinctness  of  an  image  1  What 
rays  only  does  the  screen  permit  to  ] 


OPTICS. 


159 


jformer  pencil,  but  each  finds  an  appropriate  place  of 
its  own  in  the  image,  and  all  together  make  a  faithful 
I  representation  of  the  object. 

163.  Concave  mirrors   form   images  of  objects,  by 

collecting  the  rays  from  each  point  of  the  object  into 

j  corresponding  points  in  the  image,  unaccompanied  by 

,  rays  from  any  other  quarter.     If  the  object  be  nearer 

Fig.  80. 


Fig.  81. 


•a, 


than  the  focus,  as  in  figure  80,  a  magnified  image  ap- 
pears behind  the  mirror,  and  in  its  natural  position ; 
but  if  the  object  be  between  the  focus  and  the  center, 
the  image  is  before  the  mirror,  on  the  other  side  of  the 
center,  larger  than  the  object,' 
and  inverted,  as  it  is  in  fig- 
ure 81,  where  the  small  ar- 
row, A  B,  situated  between 
the  focus  and  the  center  of 
the  mirror,  is  reflected  into 
the  image  a  b,  inverted  and 
larger  than  the  object.  These 
cases  may  be  verified  in  a 
dark  room,  by  placing  a  lamp 
at  different  distances  from  a  . 
concave  mirror.  As  such : 
mirrors  form  their  images  in 
the  air  without  any  visible 

163.  How  do  conca.ve  mirrors  form  images  1  When  the  object  is 
nearer  the  mirror  than  the  focus,  how  does  the  image  appear! 
How  when  it  is  farther  than  the  focus  *.  How  may  these  cases  be 


160 


NATURAL  PHILOSOPHY. 


support,  they  have  sometimes  been  employed  by  jug. 
glers  to  produce  apparitions  of  ghostly  figures,  drawn 
swords,  and  the  like,  which  were  made  to  appear  in 
terrific  forms,  while  the  apparatus  by  which  they  were 
produced,  was  entirely  concealed  from  the  spectators. 

Fig.  82. 


A  convex  mirror  gives  a  diminished  image  of  any  object 
placed  before  it,  representing  it  in  its  natural  position, 
and  behind  the  mirror,  as  in  figure  82. 

164.  Refraction  is  the  change  of  direction  which  light 
undergoes  by  passing  out  of  one  medium  into  another. 
When  light  passes  out  of  a  rare  me- 
dium, like  air,  into  a  dense  medium, 
like  water,  it  is  turned  toward  a  per- 
pendicular ;  when  it  passes  out  of  a 
dense  into  a  rare  medium,  it  is  turn- 
edfrom  a  perpendicular.  When  the 
ray  of  light,  B  C,  passes  out  of  air  in- 
i  to  water,  it  will  not  proceed  straight; 
in  forward  in  the  line  C  F,  but  will  go> 
1  in  the  line  C  E,  nearer  to  the  per- 
pendicular, C  H  ;  and  light  proceeding  from  an  object 
under  water  at  E  would,  on  passing  into  the  air  at  C, 
turn  from  the  perpendicular  into  the  line  C  B.  Since 

verified  1  "What  use  is  made  of  concave  mirrors  by  jugglers'?  How 
do  convex  mirrors  represent  objects  1 

164.  Define  refraction.    How  is  light  reflected  by  passing  out  of 
air  into  water— out  of  water  into  air  1    Explain  Fig.  83.    Also,  Fig.. 


II    E 


OPTICS. 


161 


Fig.  84. 


•  objects   always  appear 

|  in  the  direction  in  which 

|  the  light  finally  comes 
to  the  eye,  the  place  of 
an  image  is  changed  by 
its  light  passing  through 
a  refracting  medium  be- 
fore it  reaches  the  eye. 
Fig.  84  represents  a 
bowl  with  a  small  coin 
at  the  bottom.  An  eye 
situated  as  in  the  figure,  would  not  see  the  coin ;  but, 
on  turning  water  into  the  bowl,  the  coin  becomes  vis- 
ible at  B,  because  the  light  proceeding  from  the  coin  is 
bent  toward  the  eye  in  passing  out  of  the  water.  For 
a  similar  reason,  an  oar  in  water  appears  bent,  the  part 
immersed  being  elevated  by  refraction.  The  bottom 
of  a  shallow  river  appears  higher  than  it  really  is,  and 
people  have  been  drowned  by  attempting  to  ford  a  river 
which,  from  the  effect  of  refraction,  appeared  less  deep 
than  it  was. 

165.  The  Multiplying  Glass 
shows  as  many  images  of  an 
object  as  there  are  surfaces, 
since  each  surface  refracts  the 
light  that  falls  upon  it,  in  a 
different  angle  from  the  others ; 
of  course  the  rays  meet  the 
eye  in  the  same  number  of 
different  directions,  and  the 
object  appears  in  the  direction 
of  each.  The  candle  at  A,  Fig.  85,  sends  rays  to 
each  of  the  three  surfaces  of  the  glass.  Those  which 
fall  on  it  perpendicularly,  pass  directly  through  the 

84.   Why  an  oar  in  water  appears  bent  1   How  does  refraction  affect 
the  apparent  depth  of  a  river  1 

165.  Describe  the  multiplying  glass,  and  explain  its  effects. 
14* 


Fig.  85. 


B 


162  NATURAL  PHILOSOPHY. 


glass  to  the  eye,  without  change  of  direction,  and  form 
one  image  in  its  true  place  at  A.  But  the  rays  which 
fall  on  the  two  oblique  surfaces,  have  their  directions 
changed  both  in  entering  and  in  leaving  the  glass, 
(as  will  be  seen  by  following  the  rays  in  the  figure,) 
so  as  to  meet  the  eye  in  the  directions  of  B  and  C. 
Consequently,  images  of  the  candle  are  formed,  also, 
at  both  these  points.  A  multiplying  glass  has> 
usually  a  great  many  surfaces  inclined  to  one  another, 
and  the  number  of  images  it  forms  is  proportionally 
great. 

166.  This  property  of  light — the  power  of  having 
its  direction  changed  by  refraction — is  converted  to. 
very  important  and  inter- 
esting  uses  by  means  of 
LENSES.  A  lens  is  exem- 
plified  in  a  common  sun- 
glass,  (or  even  in  a  spec- 
tacle-glass,) and  is  either 
convex  or  concave.  Con- 
vex lenses,  like  concave 
mirrors,  collect  rays  of 
light.  In  Fig.  86,  the  parallel  rays,  A  a  and  C  cy 
are  collected  along  with  the  central  ray  (which  be- 
ing perpendicular  to  the  surfaces  of  the  lens,  suffers, 
no  refraction)  into  a  common  focus  in  F.  If  I  hold  a 
sun-glass,  or  a  pair  of  convex  spectacles  toward  the 
sun,  the  whole  beam  that  falls  upon  the  glass  will  be 
collected  into  a  small  space,  forming  a  bright  point, 
or  focus,  at  a  certain  distance  from  the  lens  on  the 
side  opposite  the  sun,  where  it  may  be  received  on  a 
screen  or  sheet  of  white  paper.  A  concave  lens,  like 

166.  To  what  important  and  interesting  uses  is  the  power  of  light 
to  undergo  refraction  converted  1  What  instruments  are  used  Tor 
this  purpose  1  What  is  the  office  of  a  convex  lens  1  Describe  Fig,. 
S6.  Examples  in  a  sun-glass  and  spectacles. 


OPTICS. 


163 


i  a  convex  mirror,  separates  rays  of  light.     Thus,  in 

Fig.  87,  the  solar  beam 
Fig.  87.  is  spread  over  a  greater 

space  on  the  screen  than 
the  size  of  the  lens,  indi- 
eating  that  the  rays  are 
separated  from  each  other 
by  passing  through  the 
lens.  Hence,  concave 
lenses  do  not  form  images 
as  convex  lenses  do,  and 
are  therefore  but  little  employed  in  the  construction  of 
optical  instruments. 

167.  A  convex  lens,  like  a  concave  mirror,  forms 
an  image  of  an  object  without,  by  collecting  all  the 
pencils  of  rays  that  proceed  from  every  point  of  the 
object  and  fall  upon  the  lens,  into  corresponding  points 
#t  the  place  of  the  image.  The  image  is  inverted 

Fig.  88. 


n 


because  the  pencils  of  rays  cross  each  other,  those 
from  the  top  of  the  object  going  to  the  bottom  of  the 
image,  and  those  from  the  bottom  going  to  the  top. 
In  the  figure,  the  central  ray  of  each  pencil  (called  the 
axis)  and  the  extreme  rays  are  represented.  The  ex- 

167.  How  does  a  convex  lens  form  an  image  1  Why  is  the  image 
inverted  1  What  is  the  axis  of  a  pencil  of  rays  7  Where  do  the 
axes  cross  each  other  7  Great  number  of  rays  that  proceed  from 
every  point  in  the  object. 


164  NATURAL  PHILOSOPHY. 

treme  rays  cross  each  other  in  the  center  of  the  lens,, 
and  thus  necessarily  produce  an  inverted  image  ;  but 
we  must  conceive  of  a  great  number  of  rays  proceed-, 
ing  from  every  point  in  the  object,  and  each  pencil- 
covering  the  whole  lens,  which  collects  them  severally 
into  distinct  points,  each  occupying  a  separate  place  ' 
in  the  image. 

168.  If  we  place  a  lamp  in  the  focus  of  a  lens,  the 
rays  that  proceed  from  it  and  pass  through  the  lens, 
go  out  parallel,  and  will  never  come  to  a  focus  on  the 
other  side,  so  as  to  form  an  image.     But  if  we  remove 
the  lamp  farther  from  the  lens,  so  as  to  make  the  rays  t 
fall  upon  the  lens  in  a  state  less  diverging,  then  it  will ; 
collect  them  into  a  distinct  image  on  the  other  side,  J 
which  image  will  be  large  in  proportion  as  it  is  morel 
distant  from  the  lens.     As   the   object   is   withdrawn 
from  the  lens,  the  image  approaches  it ;  when  they  are ) 
at  equal  distances  from  the  lens,  they  are  equal  in  size ; 
but  when  the  object  is  farther  from  the  lens  than  the  \ 
image,  the  image  is  less  than  the  object.     These  prin-  'I 
ciples  lead  to  an  understanding  of  those   interesting  f 
and  wonderful   instruments,  the  Microscope  and  the  * 
Telescope,  to  which  our  attention  will  hereafter  be  di- ' 
rected. 

SEC.  2.  Of  Colors. 

169.  The  philosophy  of  colors  has  been  unfolded 
chiefly  by  means  of  the  Prism.     A  Prism  is  a  trian- 
gular piece  of  glass,  usually  four  or  five  inches  long, 
presenting  three   plane  smooth   surfaces.     When  we 
look  through  the  prism,  all  external  objects  appear  in 

168.  How  do  the  rays  go  out  when  a  lamp  is  placed  in  the  focus  1 
How  when  the  lamp  is  farther  from  the  lens  than  the  focus  1    How 
is  the  size  of  the  image  affected  by  its  distance  from  the  lens  1   How 
is  the  image  changed  by  withdrawing  the  lamp! 

169.  By  what  instrument  has  the  philosophy  of  colors  been  unfold- 
ed 1    Define  a  prism.    What  appearances  does  it  present  when  we 


OPTICS. 


165 


the  most  brilliant  hues,  diversified  by  the  various  colors 
of  the  rainbow.  The  reason  of  this  is,  that  light  consists 
of  seven  different  colors,  which,  when  in  union  with 
each  other,  compose  white  light ;  but  when  separated, 
appear  each  in  its  own  peculiar  hue.  The  different 
colors  are  as  follows — violet,  indigo,  blue,  green,  yellow, 
orange,  red.  The  prism  separates  the  rays  of  solar 
light,  in  consequence  of  their  having  the  property  of 
I  undergoing  different  degrees  of  refraction  in  passing 
i  through  it,  the  violet  being  turned  most  out  of  its  course 
and  the  red  least,  and  all  the  others  differing  among 
themselves  in  this  respect,  as  is  shown  in  the  following 
diagram.  E  F  represents  the  window  shutter  of  a  dark 

Fig.  89. 


,^" 


room,  through  a  small  opening  in  which  a  beam  of  solar 
rays  shines.  They  fall  on  the  prism,  ABC,  and  are 
refracted,  by  which  they  are  turned  upward,  but  in 
different  degrees,  the  red  least  and  the  violet  most.  By 
this  means  they  are  separated  from  each  other,  and  lie 
one  above  another  on  the  opposite  wall,  constituting 

look  through  it  1  Why  1  Seven  colors  of  the  spectrum.  "Why 
does  the  prism  separate  the  different  colors  1  Explain  Fig.  89.  How 
may  \ve  recompose  the  spectrum  into  white  light  1 


166  NATURAL  PHILOSOPHY. 

the  beautiful  object  called  the  solar  spectrum.  We 
may  now  introduce  a  double  convex  lens  into  thq: 
spectrum,  just  behind  the  prism,  and  collect  all  the 
rays  which  have  been  separated  by  the  prism,  and  they 
will  recompose  white  light.  The  elongated  spectrum 
on  the  wall,  presenting  the  seven  primary  colors,  will 
vanish,  and  in  the  place  of  it  will  appear  a  round  image 
of  the  sun  as  white  as  snow. 

170.  We  may  now  learn  the  reason  why  so  many 
different  colors  appear  when  we  look  through  the  prism. 
The  leaves  of  a  tree,  for  example,  seem  to  send  forth 
streams  of  red  light  on  one  side  and  of  violet  on  the 
other.     The  intermediate  colors  lap  over,  and  partly 
neutralize  each  other,  while  on  the  margin  each  color 
exhibits  its  own  proper  hue. 

171.  The  rainbow  owes  its  brilliant  colors  to  the 
same  cause,  namely,  the  production  of  the  individual 
colors  that  compose  solar  light,  in  consequence  of  the 
separation  they  undergo  by  refraction  in  passing  through 
drops  of  water.     Although  drops  of  water  are  small 
objects,  yet  rays  of  light  are  still  smaller,  and  have 
abundant  room  to  enter  a  drop  of  water  on  one  side,  to 
be  reflected  from  the  opposite  surface,  and  to  pass  out 
on  the  other  side,  as  is  represented  in  the  following 
figure.     The  solar  beam  enters  the  drop  of  rain,  and , 
some  portion  (a  very  small  portion  is  sufficient)  being 
refracted  to  B,  then  reflected  and   finally  refracted 
again  in  leaving  the  drop,  is  conveyed  to  the  eye  of. 
the  spectator.     As  in  undergoing  these  two  refractions, 
some  rays  are  refracted  more  than  others,  consequently 
they  are  separated  from  each  other,  and  coming  to  the 
eye  of  the  spectator  in  this  divided  state,  produce  each 


170.  Why  do  so  many  different  colors  appear  when  we  look 
through  the  prism  1    Explain  the  appearance  of  the  leaves  of  a  tree. 

171.  To  what  does  the  rainbow  owe  its  colors  1    Explain  how  the 
separation  of  colors  is  produced.    To  what  part  of  the  bow  does  the 
line  pass  which  joins  the  sun  and  the  eye  of  the  spectator  1  How  high 


OPTICS.  167 

its  own  color.     The  spectator  stands  with  his  back  to 

ithe  sun,  and  a  straight  line  passing  from  the  sun  through 
Fig.  90. 


the  eye  of  the  spectator,  passes  also  through  the  center 
of  the  bow.  When  the  sun  is  setting,  so  that  this  line 
becomes  horizontal,  the  summit  of  the  bow  reaches  an 
altitude  of  about  42°,  and  the  bow  is  then  a  semicircle. 
When  the  sun  is  42°  high,  the  same  line  would  pass 
42°  below  the  opposite  horizon,  and  the  summit  of  the 
bow  would  barely  reach  the  horizon.  When  the  sun 
is  between  these  two  altitudes,  the  bow  rises  as  the 
sun  descends,  composing  a  larger  and  larger  part  of 
a  circle,  until,  as  the  sun  sets,  it  becomes  an  entire 
semicircle. 

172.  The  varied  colors  that  adorn  the  face  of  nature, 
as  seen  in  the  morning  and  evening  cloud,  in  the  tints  of 
flowers,  in  the  plumage  of  birds  and  wings  of  certain 
insects,  and  in  the  splendid  hues  of  the  precious  gems, 
arise  from  the  different  qualities  of  different  bodies 
in  regard  to  the  power  of  refracting  or  of  reflecting 
light.  When  a  substance  reflects  all  the  prismatic 
rays  in  due  proportion,  its  color  is  white  ;  when  it 
absorbs  them  all,  its  color  is  black  ;  and  its  color  is  blue, 

does  the  top  of  the  bow  reach  when  the  sun  is  setting  1  "Where  ia 
it  when  the  sun  is  42°  above  the  western  horizon  1  When  the  sun 
is  between  these  two  points  1 


168  NATURAL  PHILOSOPHY. 

green,  or  yellow,  when  it  happens  to  reflect  one  of  these 
colors,  and  to  absorb  all  the  others  of  the  spectrum. 
These  hues  are  endlessly  varied  by  the  power  natural  i 
bodies  have  of  reflecting  a  mixture  of  some  of  the^ 
primary  colors  to  the  exclusion  of  others,  every  new- 
proportion  producing  a  different  shade. 

SEC.  3.  Of  Vision. 

173.  Whenever  we  admit  into  a  dark  room  through  : 
an  opening  in  the  shutter,  light  reflected  from  various*- 
objects  without,  an  inverted  picture  of  these  objects? 
will  be  formed  on  the  opposite  wall.     A  room  fitted  for- 
exhibiting  such  a  picture,  is  called  a  Camera  Olscura* 
In  a  tower  which  has  a  window  opening  toward  the 
east,    upon    a    beautiful    public    square,    containing 
churches  and  other  public  buildings,  and  numerous 
trees,  and  the  various  objects  of  a  populous  city,  at 
little  dark  chamber  is  fitted  up  for  a  camera  obscura, 
having  a  white  concave  stuccoed  wall  opposite  to  the 
window,  ten  feet  from  it,  and  all  the  other  parts  of  the 
room  painted  black.     The  afternoon,  when  the  sun  is;< 
shining  bright  in  the  west,  and  all  objects  seen  to  the 
east  present  their  enlightened  sides  toward  the  window, 
is  the  time  for  forming  the  picture.     For  this  purpose, 
a  round  hole  about  three  inches  in  diameter,  is  prepared 
in  the  shutter,  which  admits  the  only  light  that  can| 
enter   the   room.      The   room   is  made  black  every- 
where except  the  wall  that  is  to  receive  the  picture/ 
otherwise  light  would  be  reflected  from  different  parts 
of  the  room  upon  the  picture  ;  whereas  it  is  essential 
to  its  distinctness,  that   the  image  should    be  unac- 

172.  How  are  the  colors  of  natural  objects  produced  1    When  is 
the  color  white,  or  red,  or  blue  1    How  are  the  colors  varied  1 

173.  How  may  a  picture  be  formed  in  a  dark  room  1    What  is  it 
called  1    Describe  the  Camera  Obscura  mentioned.    Wnen  is  the 
time  for  forming  the  picture  1    Why  is  the  room  painted  black, 
except  the  wall  opposite  the  window  1 


OPTICS.  169 

rjompanied  by  light  from  any  other  source.  The  wall 
[hat  is  to  receive  the  picture  is  made  concave,  so  that 
bvery  part  of  it  may  be  equally  distant  from  the  orifice 
in  the  shutter. 

j  174.  We  now  close  the  shutter,  and  instantly  there 
Appears  on  the  opposite  wall  a  large  picture,  repre- 
senting all  the  varied  objects  of  the  landscape  seen 
:>om  the  window,  as  churches,  houses,  trees,  men  and 
ivomen,  carriages  and  horses,  and  in  short  every  thing 
;hat  is  in  view  of  the  window,  including  the  blue  sky, 
ind  a  few  white  clouds  that  are  sailing  through  it. 
Sach  is  represented  in  its  proportionate  size  and  color, 
Smd  if  it  is  moving,  in  its  true  motion.  Two  circum- 
stances, only,  impair  the  beauty  of  the  picture  ;  one 
s,  that  it  is  not  perfectly  distinct,  the  other,  that  it  is 
inverted — the  trees  appear  to  grow  downward,  and  the 
j>eople  to  walk  with  their  feet  above  their  heads.  The 
hicture  appears  indistinct,  because  the  opening  in  the 
shutter  is  so  large  that  rays  coming  from  different  ob- 
jects fall  upon  the  picture  and  mix  together,  whereas 
?ach  point  in  the  image  must  be  formed  alone  of  rays 
poming  from  a  corresponding  point  in  the  object.  We 
ivill  therefore  diminish  the  size  of  the  opening  by  cov- 
ering it  with  a  slide  containing  several  holes  of  differ- 
ent sizes.  We  will  first  reduce  the  diameter  to  an 
inch.  The  picture  is  now  much  more  distinct,  but  yet 
ftot  perfectly  well  defined.  We  will  therefore  move 
the  slide,  and  reduce  the  opening  to  half  an  inch. 
Now  the  objects  are  perfectly  well  defined,  for  through 
5o  small  an  openjng  none  but  the  central  ray,  or  axis, 
pf  each  pencil  can  enter,  and  each  axis  will  strike  the 
opposite  wall  in  a  point  distinct  from  all  the  rest.  But 
though  the  picture  is  no  longer  confused,  yet  it  lacks 
brightness,  for  so  few  rays  scattered  over  so  large  a 

174.    On  closing  the  shutter,  what  appearances  present  them- 
selves 1   What  two  circumstances  impair  the  beauty  of  the  picture  1 
Why  indistinct— How  rendered  more  distinct— Why  well  defined 
15 


170 


NATURAL   PHILOSOPHY. 


Fig.  91. 


surface,  are  insufficient  to  form  a  bright  image.  Wo 
will  now  remove  the  slide,  open  the  original  orifice  of 
three  inches,  which  lets  in  a  great  abundance  of  light, 
and  we  will  place  immediately  before  the  orifice, 
(within  the  room,)  a  convex  lens  of  ten  feet  focus, 
which  will  collect  all  the  scattered  rays  into  separate 
foci,  and  thus  form  a  picture  at  once  distinct  and  bright, 
so  that  the  most  delicate  objects  without,  as  the  trembling 
of  the  leaves  of  the  trees,  and  the  minutest  motions  of 
animals,  are  all  very  plainly  discernible.  Only  one 
thing  is  wanting  to  make  the  picture  perfect,  and  that 
is,  to  turn  it  right  side  upward.  This  may  be  done, 
and  is  done  in  some  forms  of  the  camera  obscura  ;  but 
for  our  present  purpose,  which  is  to  illustrate  the  prin- 
ciples of  the  eye,  where  the  image  formed  is  also  in- 
verted, it  is  better  as  it  is. 

175.  The  eye  is  a  ca- 
mera obscura,  and  the 
analogy  between  its  prin- 
cipal parts  and  the  contri- 
vances employed  to  form 
a  picture  of  external  ob- 
jects, as  in  the  foregoing 
dark  chamber,  will  appear 
very  striking  on  compari- 
son. Figure  91  represents 
the  human  eye,  which  is  a 
circular  chamber,  colored 
black  on  all  sides  except  the  back  part,  called  the  re- 
tina, which  is  a  delicate  white  membrane,  like  the 
finest  gauze,  spread  to  receive  the  image.  The  front 
part  of  the  eye,  A,  is  a  lens  of  a  shape  exactly  adapted 
to  the  purpose  it  is  intended  to  serve,  which  projects 

when  the  orifice  is  small — What  does  the  picture  now  lack  1  How 
to  make  it  at  once  well  defined  and  bright  1 

175.  Analogy  of  the  eye  to  the  Camera  Obscura.    Describe  ..the 
eye  from  Fig.  91. 


OPTICS.  171 

forward  so  as  to  receive  the  light  that  comes  in  side- 
wise,  and  guides  it  into  the  eye.  The  pupil  is  an 
opening  between  c  and  c,  like  the  opening  in  the  win- 
dow shutter,  just  behind  which  is  a  convex  lens,  B, 
which  collects  all  the  scattered  rays,  and  brings  each 
pencil  to  a  separate  focus,  where  they  unite  in  forming 
a  bright  and  beautifully  distinct  image  of  all  external 
objects.  O  represents  the  optic  nerve,  by  which  the 
sensations  made  on  the  retina  are  conveyed  to  the  brain. 
The  substances  with  which  the  several  parts  of  the  eye, 
A,  B,  and  C,  are  filled,  are  limpid  and  transparent,  and 
purer  than  the  clearest  crystal. 

176.  It  is  essential  to  distinct  vision,  that  the  rays 
which  enter  the  eye  should  be  brought  accurately  to 
a  focus  at  the  place  of  the  retina  ;  and  in  ninety-nine 
cases  out  of  a  hundred,  this  adjustment  is  perfect.  But 
in  a  few  instances,  the  lens,  B,  called  the  crystalline 
humor,  is  too  convex,  and  then  the  image  is  formed  be- 
fore it  reaches  the  retina.  This  is  the  case  with  near- 
sighted people.  Their  eyes  are  too  convex  ;  but  by 
wearing  a  pair  of  concave  spectacles,  they  can  destroy 
the  excess  of  convexity  in  the  eye,  and  then  the  crys- 
talline lens  will  bring  the  light  to  a  focus  on  the  retina 
and  the  sight  will  be  distinct.  Sometimes,  particularly 
as  old  age  advances,  the  crystalline  lens  becomes  less 
convex,  and  does  not  bring  the,  rays  to  a  focus  soon 
enough,  but  they  meet  the  retina  before  they  have  come 
accurately  to  a  focus,  and  form  a  confused  image.  In 
this  case  a  pair  of  convex  spectacles  aids  the  crystal- 
line lens,  and  both  together  cause  the  image  to  fall  ex- 
actly on  the  retina.  As  a  piece  of  mechanism,  the  eye 
is  unequalled  for  its  beauty  and  perfection,  and  no  part 
of  the  creation  proclaims  more  distinctly  both  the  ex- 
istence and  the  wisdom  of  the  Creator. 

176.  What  is  essential  to  distinct  vision  1  Imperfection  when  the 
crystalline  lens  is  too  convex — how  remedied — also  when  not  con- 
vex enough — remedy  1  Perfection  of  the  eye. 


172  NATURAL  PHILOSOPHY. 

SEC.  4.  Of  the  Microscope. 

177.  The  Microscope  is  an  optical  instrument,  de- 
signed to  aid  the  eye  in  the  inspection  of  minute  objects. 
The  simplest  microscope  is  a  convex  lens,  like  a  spec- 
tacle glass.     This,  when  applied  to  small  objects,  as 
the  letters  of  a  book,  renders  them  both  larger  and  more 
distinct.     When  an  object  is  brought  nearer  and  near- 
er to  the  eye,  we  finally  reach  a  point  within  which 
vision  begins  to  grow  imperfect.     That  point  is  called 
the  limit  of  distinct  vision.     Its  distance  is  about  five 
inches.     If  the  object  be  brought  nearer  than  this  dis- 
tance, the  rays  come  to  the  eye  too  diverging  for  the 
lenses  of  the  eye  to  bring  them  to  a  focus  soon  enough, 
so  as  to  make  their  image  fall  exactly  on  the  retina. 
Moreover,  the  rays  which  proceed  from  the  extreme 
parts  of  the  object,  meet  the  eye  too  obliquely  to  be 
brought  to  the  same  focus  with  those  rays  which  meet 
it  more  directly,  and  hence  contribute  only  to  confuse 
the  picture. 

178.  We  may  verify  these   remarks   by  bringing  ' 
gradually  toward  the  eye  a  printed  page  with  small  ! 
letters.     When   the    letters    are  within  two  or  three  ; 
inches  of  the  eye,  they  are  blended  together  and  noth-  j 
ing  is  seen  distinctly.     If  we  now  make  a  pin-hole  - 
through  a  piece  of  paper,  and  look  at  the  same  letters  ; 
through  this,  we  find  them  rendered  far  more  distinct  • 
than  before  at  near  distances,  and  larger  than  ordina- 
ry.    Their  greater  distinctness  is  owing  to  the  exclu- 
sion of  those  oblique  rays  which,  not  being  brought  by 
the  eye  to  the  same  focus  with  the  central  rays,  only 
tend  to  confuse  the  image  formed  by  the  latter.     As 

177.  Define  the  microscope.    What  is  the  simplest  form  of  the 
microscope  1    Its  effect  upon  the  letters  of  a  book.    What  is  the 
limit  of  distinct  vision  1    Why  do  objects  appear  indistinct  when 
nearer  than  this  1 

178.  Example  in  a  printed  page — Appearance  through  a  pin-hole. 
To  what  is  the  greater  distinctness  owing'! — The  increased  brightness] 


OPTICS.  173 

only  the  central  rays  of  each  pencil  can  enter  so  small 
an  orifice,  the  picture  is  made  up  chiefly  of  the  axes 
of  all  the  pencils.  These  occupy  each  a  separate 
point  in  the  image,  a  point  where  no  other  rays  can 
reach.  The  increased  magnitude  of  the  letters  is  owing 
to  their  being  seen  nearer  than  ordinary,  and  thus  un- 
der a  greater  angle,  and  of  course  magnified. 

179.  A  convex  lens  acts  much  on  the  same  princi- 
ples, but  is  still  more  effectual.  It  does  not  exclude 
the  oblique  rays,  but  it  diminishes  their  obliquity  so 
much  as  to  enable  the  eye  to  bring  them  to  a  focus, 
at  the  distance  of  the  retina,  and  thus  makes  them  con- 
tribute to  the  brightness  of  the  picture.  The  object  is 
magnified,  as  before,  because  it  is  seen  nearer,  and 
consequently  under  a  larger  angle,  so  that  the  eye  can 
distinctly  recognise  minute  portions  of  the  object, 
which  were  before  invisible,  because  they  did  not  oc- 
cupy a  sufficient  space  on  the  retina.  Lenses  have 
greater  magnifying  power  in  proportion  as  the  convex- 
ity is  greater,  and  of  course  the  focal  distance  less. 
Since  the  magnifying  power  of  the  microscope  arises 
,  from  its  enabling  us  to  see  objects  nearer  and  under  a 
larger  angle,  that  power  is  increased  in  proportion  as 
the  focal  distance  is  less  than  the  limit  of  distinct  vis- 
ion. The  latter  being  five  inches,  a  lens  which  has 
a  focal  distance  of  one  inch,  by  enabling  us  to  see  the 
object  five  times  nearer,  enlarges  its  length  and  breadth 
each  five  times,  and  its  surface  twenty-five  times. 
Lenses  have  been  made  capable  of  affording  a  distinct 
image  of  very  minute  objects,  when  their  focal  dis- 
tances were  only  one-sixtieth  of  an  inch.  In  this  case, 
the  magnifying  power  would  be  as  one-sixtieth  to  five ; 


179.  Explain  the  mode  in  which  a  convex  lens  acts.  Why  it 
makes  objects  appear  brighter  and  larger.  What  lenses  have  the 
greatest  magnifying  power  1  Power  of  a  lens  of  one  inch  focus — 
of  one  sixtieth  of  an  inch  focus. 


174 


NATURAL  PHILOSOPHY. 


or  it  would  magnify  the  length  and  breadth  each  300 
times,  and  the  surface  90,000  times. 

180.  The  Magic  Lantern  and  Solar  Microscope  owe 
their  astonishing  effects  to  the  magnifying  power  of  a 
simple  lens.  When  the  image  so  much  exceeds  the 
object  in  magnitude,  were  the  object  only  enlightened 
by  the  common  light  of  day,  when  it  came  to  be  diffu- 
sed over  so  great  a  space,  it  would  be  very  feeble,  and 
the  image  would  be  obscure  and  perhaps  invisible. 
The  two  instruments  just  named,  have  each  an  appa- 
ratus connected  with  the  magnifying  lens,  which  serves 
to  illuminate  the  object  highly,  so  that  when  the  rays 
that  proceed  from  it  and  form  the  enlarged  image  are 
spread  over  so  great  a  space,  they  may  still  be  suffi- 
cient to  render  the  image  bright  and  distinctly  visible. 
Fig.  92. 


181.  In  the  Magic  Lantern,  the  illumination  is  af- 
forded by  a  lamp,  "the  light  of  which  is  reflected  from 
a  concave  mirror  placed  behind  it,  which  makes  the 
light  on  that  side  return  to  unite  with  the  direct  light 

180.  To  what  are  the  effects  of  the  Magic  Lantern  and  the  Solar 
Microscope  owing  1    Use  of  all  the  other  parts  of  the  apparatus,  ex- 
cept the  magnifier  1 

181.  How  is  the  illumination  effected  in  the  magic  lantern  1    De- 
scribe Fig.  92.    What  sorts  of  objects  are  exhibited  1 


OPTICS.  175 

of  the  lamp,  so  that  both  fall  on  a  large  lens  which  col- 
lects them  upon  the  object,  thus  strongly  illuminating 
it.  The  foregoing  diagram  exhibits  such  a  lantern, 
where  the  concave  mirror  behind  is  seen  to  reflect 
back  the  light  to  unite  with  that  which  proceeds  di- 
rectly from  the  lamp,  so  that  both  fall  on  the  large  con- 
vex lens  at  C,  which  collects  them  upon  the  object  at 
B.  This  is  usually  painted  in  transparent  colors  on 
glass,  and  may  be  a  likeness  of  some  individual,  small 
in  the  picture,  but  when  magnified  by  the  lens,  A,  and 
the  image  thrown  on  a  screen  or  wall,  F,  will  appear 
as  large  as  life,  and  in  strong  colors ;  or  the  objects 
may  be  views  of  the  heavenly  bodies,  which  are  thus 
often  rendered  very  striking  and  interesting  ;  or  they 
may  illustrate  some  department  of  natural  history,  as 
birds,  fishes,  or  plants. 

182.  The  Solar  Microscope  is  the  same  in  princi- 
ple with  the  Magic  Lantern,  but  the  light  of  the  sun 
instead  of  that  of  a  lamp  is  employed  to  illuminate  the 
object.  As  a  powerful  light  may  thus  be  commanded, 
very  great  magnifiers  can  be  employed ;  for  if  the  ob- 
ject is  highly  illuminated,  the  image  will  not  be  feeble 
or  obscure  when  spread  over  a  great  space.  By  means 
of  this  instrument,  the  eels  in  vinegar,  which  are  usu- 
ally so  small  as  to  be  invisible  to  the  naked  eye,  may 
be  made  to  appear  six  feet  in  length,  and,  as  their  mo- 
tions as  well  as  dimensions  are  magnified,  they  will 
appear  to  dart  about  with  surprising  velocity.  The 
finest  works  of  art,  when  exhibited  in  this  instrument, 
appear  exceedingly  coarse  and  imperfect.  The  eye 
of  a  finished  cambric  needle  appears  full  of  rough  pro- 
jections ;  the  blade  of  a  razor  looks  like  a  saw ;  and 
the  finest  muslin  exhibits  threads  as  large  as  the  cable 
of  a  ship.  Thus,  the  small  and  almost  invisible  insect 

182.  Solar  Microscope,  how  it  differs  from  the  Magic  Lantern — 
Why  greater  magnifiers  can  be  used — appearance  of  the  eels  in 
vinegar.  How  do  the  works  of  art  appear  *?  How  small  insects  1 


176 


NATURAL  PHILOSOPHY. 


represented  in  figure  93,  gives  out,  when  illuminated, 
so  few  rays,  that  when  spread  over  the  large  surface 
of  the  image,  the  light  would  be  too  feeble  to  render 
the  image  visible ;  but,  on  strongly  illuminating  the 

Fig.  93. 


insect  by  concentrating  upon  it  a  large  beam  of  the 
sun's  light,  the  image  becomes  distinct  and  beautiful, 
although  perhaps  a  million  times  as  large  as  the  object. 
Even  the  minute  parts  of  the  insect,  as  the  hairs  on  the  i 
legs,  are  revealed  to  us  by  the  microscope. 

SEC.  5.  Of  the  Telescope. 

183.  The  Telescope  is  an  instrument  employed  for 
viewing  distant  objects.     It  aids  the  eye  in  two  ways ;  . 
first,  by  enlarging  the  angle  under  which  objects  are  •; 
seen,  and,  secondly,    by  collecting  and  conveying  to  : 
the  eye  a  much  larger  amount  of  the  light  that  pro- 
ceeds from  the  object,  than  would  enter  the  naked  pu- 
pil.     We  first  form  an  image  of  a  distant  object,  the 

183.  Define  the  Telescope.  In  what  two  ways  does  it  aid  the  eye  1 


OPTICS.  177 

moon,  for  example,  and  then  magnify  that  image  Try  a 
microscope.  The  image  may  be  formed  either  by  a 
concave  mirror  or  a  convex  lens,  for  both,  as  we  have 
seen,  form  images.  Although  we  cannot  go  to  distant 
objects,  as  the  moon  and  planets,  so  as  to  view  them 
under  the  enlarged  dimensions  in  which  they  would 
then  appear,  yet  by  applying  a  microscope  to  the  image 
of  one  of  those  bodies,  we  may  make  it  appear  as  it 
would  do  were  we  to  come  much  nearer  to  it.  To  ap- 
ply a  microscope  which  magnifies  a  hundred  times,  is 
the  same  thing  as  to  approach  a  hundred  times  nearer 
to  the  body. 

Fig.  94. 


184.  Let  A  B  C  D  represent  the  tube  of  the  tele- 
scope.  At  the  front  end,  or  the  end  which  is  directed 
toward  the  object,  (which  we  will  suppose  to  be  the 
moon,)  is  inserted  a  convex  lens,  L,  which  receives  the 
rays  of  light  from  the  moon,  and  collects  them  into  the 
focus  at  a,  forming  an  image  of  the  moon.  This 
image  is  viewed  by  a  magnifier  attached  to  the  end, 
B  C.  The  lens,  L,  is  called  the  object-glass,  and  the 
microscope,  in  B  C,  the  eye-glass.  A  few  rays  of  light 
only  from  a  distant  object,  as  a  star,  can  enter  so  small 

State  the  main  principle  of  this  instrument.    How  may  the  image 
be  formed  1    How  it  brings  objects  nearer  to  us. 

184.  Describe  the  telescope  as  represented  in  Fig.  94.  Point  out 
the  object-glass,  and  the  eye-glass.  What  is  the  use  of  a  large  ob- 


178  NATURAL   PHILOSOPHY. 

a  space  as  the  pupil  of  the  eye  :  but  a  lens  one  foot  in 
diameter  will  collect  a  beam  of  light  equal  to  a  cylin- 
der of  the  same  dimensions,  and  convey  it  to  the  eye. 
The  object-glass  merely  forms  an  image  of  the  object, 
but  does  not  magnify ;    the   microscope   or  eye-glass 
magnifies.     By  these  means,  many  obscure  celestial 
objects  become  distinctly  visible,  which  would  other- 
wise  be  too  minute,  or  not  sufficiently  luminous,  to  be 
seen  by  us.     A  telescope  like  the  foregoing,  having 
simply  an  object-glass  and  an  eye-glass,  inverts  ob- 
jects, since  the  rays  cross  each  other  before  they  form  j 
the  image.     By  employing  more  lenses,   it   may  be  I 
turned  back  again,  so  as  to  appear  in  its  natural  posi-, 
tion,  as  is  usually  done  in  spy-glasses,  or  the  smaller  •, 
telescopes  used  in  the  daytime.     But  since  every  lens/ 
absorbs  and  extinguishes  a  certain  portion  of  the  light, 
and  since,  in  viewing  the  heavenly  bodies,  we  usually 
wish  to  save  as  much  of  the  light  as  possible,  astro- 
nomical  telescopes   are    constructed   with  these   two 
glasses  only. 

185.  Instead  of  the  convex  object-glass,  we  may 
employ  the  concave  mirror  to  form  the  image.  When 
the  lens  is  used,  the  instrument  is  called  a  refracting 
telescope ;  when  a  concave  mirror  is  used,  it  is  called 
a  reflecting  telescope.  Large  reflectors  are  more  easily 
made  than  large  refractors,  since  a  concave  mirror 
may  be  made  of  any  size  ;  whereas,  it  is  very  difficult 
to  obtain  glass  that  is  sufficiently  pure  for  this  purpose 
above  a  few  inches  in  diameter,  although  Refractors 
are  more  perfect  instruments  than  Reflectors,  in  pro- 
portion to  their  size.  Sir  William  Herschel,  a  great 
astronomer  of  England,  of  the  last  century,  made  a 


ject-glass  1  "Which  glass  collects  the  light — which  magnifies  1  Can 
the  image  be  made  "to  appear  erect  1  "Why  not  done  in  the  astro- 
nomical telescope  1 

185.  Point  out  the  distinction  between  refracting  and  reflecting 
telescopes.    Give  an  account  of  Herschel's  great  telescope. 


OPTICS.  179 

reflecting  telescope  forty  feet  in  length,  with  a  concave 
mirror  more  than  four  feet  in  diameter.  The  mirror 
alone  weighed  nearly  a  ton.  So  large  and  heavy  an 
instrument  must  require  a  vast  deal  of  machinery  to 
work  it  and  keep  it  steady  ;  and  accordingly,  the  frame- 
work surrounding  it  was  formed  of  heavy  timbers,  and 
resembled  the  frame  of  a  house.  When  one  of  the 
largest  of  the  fixed  stars,  as  Sirius,  was  entering  the 
field  of  this  telescope,  its  approach  was  announced  by 
a  bright  dawn,  like  that  which  precedes  the  rising  sun; 
and  when  the  star  itself  entered  the  field,  the  light  was 
too  dazzling  to  be  seen  without  a  colored  glass  to  pro- 
tect the  eye. 

The  telescope  has  made  us  acquainted  with  innu- 
merable worlds,  many  of  which  are  fitted  up  in  a  style 
of  far  greater  magnificence  than  our  own.  To  the  in- 
teresting and  ennobling  study  of  these,  let  us  next  di- 
rect our  attention. 


RUDIMENTS 

OP 

NATURAL  PHILOSOPHY  AND  ASTRONOMY. 


PART  II. 

ASTRONOMY. 

CHAPTER  I. 
DOCTRINE    OF    THE    SPHERE. 

DEFINITIONS DIURNAL   REVOLUTIONS. 

186.  ASTRONOMY  is  that  science  which  treats  of  th 
heavenly   bodies.     More   particularly,  its   object  is  t 
teach  what  is  known  respecting  the  Sun,  Moon,  Planets 
Comets,  and    Fixed   Stars ;    and  also  to   explain  th 
methods  by  which  this  knowledge  is  acquired. 

187.  Astronomy  is  the  oldest  science  in  the  world 
but  it  was  cultivated  among  the  ancients  chiefly  for  th 
purposes  of  Astrology.     Astrology  was  the  art  of  fore 
telling  future  events  ly  the  stars.     Its   disciples  pro 
fessed  especially  to  be  able  to  tell  from  the  appearance 
of  the  stars  at  the  time  of  any  one's  birth,  what  woulc 
be  his  course  and  destiny  through  life ;  and,  respecting 
any   country,    and    public    events,    what   would    be 
their  fate,  what  revolutions  they  would  undergo,  wha 
wars  and  other  calamities  they  would  suffer,  or  wha 
good  fortune    they   would  experience.     Visionary  as 

186.  Define  Astronomy— "What  is  its  object* 

187  Antiquity  of  the  science—For  what  purpose  vfos  it  cultivate* 


DOCTRINE    OF   THE    SPHERE.  181 

this  art  was,  it  nevertheless  led  to  the  careful  obser- 
vation and  study  of  the  heavenly  bodies,  and  thus  laid 
the  foundations  of  the  beautiful  temple  of  modern  as- 
tronomy. 

188.  Astronomy  is  a  delightful  and  interesting  study, 
when  clearly  understood  ;  but  it  is  very  necessary  to  a 
clear  understanding  of  it,  that  the  learner  should  think 
for  himself,  and  labor  to  form  an  idea  in  his  mind  of 
the  exact  meaning  of  all  the  circles,  lines,  and  points 
of  the  sphere,  as  they  are  successively  defined  ;  and 
if  any  thing  at  first  appears  obscure,  he  may  be  assured 
that  by  patient  thought  it  will  clear  up  and  become 
easy,  and  then  he  will  understand  the  great  machinery 
of  the  heavens  as  easily  as  he  does  that  of  a  clock. 
"  Patient  thought,"  was  the  motto  of  Sir  Isaac  New- 
ton,  the  greatest  astronomer  that  ever  lived  ;  and  no 
other  way  has  yet  been  discovered  of  obtaining  a  clear 
knowledge  of  this  sublime  science. 

189.  Let  us  imagine  ourselves  standing  on  a  huge 
ball,  (for  such  is  the  earth,)  in  a  clear  evening.     Al- 
though the  earth  is  large,  compared  with  man  and  his 
works,  yet  it  is  very  small,  compared  with  the  vast 
extent  of  the  space  in  which  the  heavenly  bodies  move. 
When  we  look  upward  and  around  us  at  the  starry 
heavens,  we  must  conceiv9  of  ourselves  as  standing 
on  a  small  ball,  which  is  encircled  by  the  stars  on  all 
sides  of  it  alike,  as  is  represented  over  the  leaf;  and 
we  must  consider  ourselves  as  bound  to  the  earth  by 
an  invisible   force,  (gravity,)  as  truly  as  though  we 
were  lashed  to  it  with  cords.     We  are,  therefore,  in 
no  more  danger  of  falling  off,  than  needles  are  of  fall- 


amongthe  ancients'?    Define  astrology.    What  did  its  disciples  pro- 
fess 1    To  what  good  did  it  lead  * 

188.  What  is  necessary  to  a  clear  understanding  of  Astronomy  1 
What  was  the  motto  of  Newton  1 

189.  Where  shall  we  imagine  ourselves  standing  1    What  is  said 
of  the  size  of  the  earth  1    Are  persons  on  the  opposite  side  of  the 

16 


182 


ASTRONOMY. 


ing  from  a  magnet  or  loadstone,  when  they  are  attached 
to  it  on  all  sides.  We  must  thus  familiarize  ourselves 
to  the  idea  that  up  and  down  are  not  absolute  directions 


in  space,  but  we  must  endeavor  to  make  it  seem  to  us 
up  in  all  directions  from  the  center  of  the  earth,  and 
down  on  all  sides  toward  the  center.  If  people  on  the 
opposite  side  of  the  globe  seem  to  us  to  have  their 
heads  downward,  we  seem  to  them  to  have  ours  in  the 
same  position  ;  and,  twelve  hours  hence,  we  shall  be 
in  their  situation  and  they  in  ours.  We  see  but  half  j 
the  heavens  at  once,  because  the  earth  hides  the  other 
part  from  us  ;  but  if  we  imagine  the  earth  to  grow  less 
and  less  until  it  dwindles  to  a  point,  so  as  not  to  ob- 
struct our  view  in  any  direction,  then  we  should  see 
ourselves  standing  in  the  middle  of  a  vast  starry  sphere, 
encompassing  us  alike  on  all  sides.  It  is  such  a  view 


earth  in  danger  of  falling  offl  What  idea  must  we  form  of  up  and 
down  ?  How  should  we  view  the  heavens  if  the  earth  were  so 
small  as  not  to  obstruct  our  view "? 


DOCTRINE  OF  THE  SPHERE.  183 

of  the  heavens  that  the  astronomer  has  continually  in 
the  eye  of  his  mind. 

190.  We  are  apt  to  bring  along  with  us  the  first 
impressions  of  childhood  ;  namely,  that  the  sun,  moon, 
and  stars,  are  all  fixed  on  the  surface  of  the  sky,  which 
we  imagine  to  be  a  real  surface,  like  that  of  an  arched 
ceiling  ;  but  it  is  time  now  to  dismiss  such  childish  no- 
tions, and  to  raise  our  thoughts  to  more  just  views  of 
the  creation.     Our  eyesight  is  so  limited  that  we  can- 
not   distinguish   between    different-  distances,    except 
for  a  moderate  extent ;  beyond,  all  objects  seem  to  us 
at  the  same  distance,  whether  they  are  a  hundred  or  a 
million  miles  off.     The  termination  of  this  extent  of 
our  vision  being  at  equal  distances  on  all  sides  of  us, 
we  appear  to  stand  under  a  vast  dome,  which  we  call 
the  sky.     The  azure  color  of  the  sky,  when  clear,  is 
nothing  else  than  that  of  the  atmosphere  itself,  which, 
though  colorless  when  seen  in  a  small  volume,  betrays 
a  hue  peculiar  to  itself  when  seen  through  its  whole 
extent.     Were  it  not  for  the  atmosphere,  the  sky  would 
appear  black,  and  the  stars  would  seem  to  be  so  many 
gems  set  in  a  black  ground. 

191.  For  the  purpose  of  determining  the  relative 
situation  of  places,  both  on  the  earth  and  in  the  heav- 
ens, the  various  circles  of  the  sphere  are  devised  ; 
but  before  contemplating  the  sphere  marked  up  as  ar- 
tificial representations  of  it  are,  we  must  think  of  our- 
selves as  standing  on  the  earth,  as  on  a  point  in  the 
midst  of  boundless  space,  and  see,  with  our  mental  eye, 
the  pure  sphere  of  the  heavens,  undefaced  with  any 
such  rude  lines.     If  we  could  place  ourselves  on  any 

190.  What  erroneous  conceptions  are  we  apt  to  form  in  childhood 
of  the  sun,  moon,  and  stars  *    Impossibility  of  distinguishing  differ- 
ent distances  by  the  eye.    Under  what  do  we  appear  to  stand  1    To 
what  is  the  blue  color  of  the  sky  owing  1 

191.  For  what  purpose  are  the  circles  of  the  sphere   devised  1. 
What  must  we  do  before  studying  the  artificial  representations  of 
the  sphere  1    If  we  could  stand  on  one  of  the  stars,  what  should  we 


184 


ASTRONOMY. 


.  96. 


one  of  the  stars,  we  should  see  a  starry  firmament  over 
our  heads,  similar  to  that  we  see  now.     But  although 
we  obtain  the  most  correct  and  agreeable,  as  well  as   : 
the  most  sublime  views  of  the  heavenly  bodies,  when   ;; 
we  think  of  them  as  they  are  in  nature  —  bodies  scat-  \\ 
tered  at  great  distances  from  each  other,  through  bound.    | 
less  space  —  yet  we  cannot  make  much  progress  in  the    ; 
science  of  astronomy,  unless  we  learn  the  artificial  di- 
visions of  the  sphere.     Let  us,  therefore,  now  turn  our 
attention  to  these. 

192.  The  definitions  of  the  different  lines,  points,    I 
and  circles,  which   are   used   in   astronomy,  and  the    j 
propositions  founded  on  them,  compose  the  doctrine  of 
the  sphere.     Before  these  definitions  are  given,  let  us 
attend  to  a  few  particulars  respecting  the  method  of 
measuring  angles.      (See  Fig.   96.)     A  line  drawn 

from  the  center  to  the 
circumference  of  a  cir- 
cle, is  called  a  radius,  as 
C  D,  C  B,  or  C  K.  Any 
part  of  the  circumfer- 
ence of  a  circle  is  called 
an  arc,  as  A  B,  or  B  D. 
An  angle  is  measured  by 
an  arc,  included  between 
two  radii.  Thus,  in  fig- 
ure 96,  the  angle  in- 
cluded between  the  two 
radii,  C  A  and  C  B,  that 
is,  the  angle  A  C  B,  is 
measured  by  the  arc  A  B.  Every  circle  is  divided 
into  360  equal  parts,  called  degrees  ;  and  any  arc,  as 
A  B,  contains  a  certain  number  of  degrees,  according 

see  1    When  do  we  obtain  the  most  agreeable  and  sublime  views  of 
the  heavenly  bodies'?    What  else  is  necessary  to  our  progress  1 

192.  Define  the  doctrine  of  the  sphere.  What  is  the  radius  of  a  cir 
cle—  an  arc—  an  angle  1    Explain  by  Fig.  96.    Into  how  many  de- 


DOCTRINE    OF    THE    SPHERE.  185 

its  length.  Thus,  if  an  arc,  A  B,  contains  40 
degrees,  then  the  opposite  angle  is  said  to  be  an  angle 
of  40  degrees,  and  to  be  measured  by  A  B.  But  this 
arc  is  the  same  part  of  the  smaller  circle  that  E  F  is 
of  the  greater.  The  arc  A  B,  therefore,  contains  the 
same  number  of  degrees  as  the  ipirc  E  F,  and  either 
may  be  taken  as  the  measure  of  the  angle  A  C  B.  As 
the  whole  circle  contains  360  degrees,  it  is  evident  that 
the  quarter  of  a  circle,  or  quadrant,  contains  90  degrees, 
and  that  the  semi-circle  contains  180  degrees. 

193.  A  section  of  a  sphere,  cut  through  in  any 
direction,  is  a  circle.  Great  circles  are  those  which 
pass  through  the  center  of  a  sphere,  and  divide  it  into 
two  equal  hemispheres.  Small  circles  are  such  as  do 

•  not  pass  through  the  center,  but  divide  the  sphere  into 
two  unequal  parts.  This  distinction  may  be  easily 
exemplified  by  cutting  an  apple  first  through  the  center, 
and  then  through  any  other  part.*  The  first  section 
will  be  a  great,  and  the  second  a  small  circle.  The 

'  axis  of  a  circle  is  a  straight  line  passing  through  its 
center  at  right  angles  to  its  plane.  If  you  cut  a  circle 
out  of  pasteboard,  and  thrust  a  needle  through  the  center, 
perpendicularly,  it  will  represent  the  axis  of  the  circle. 
The  pole  of  a  great  circle,  is  the  point  on  the  sphere 
where  its  axis  cuts  through  the  sphere.  Every  great 
circle  has  two  poles,  each  of  which  is  everywhere  90 
degrees  from  that  circle.  All  great  circles  of  the  sphere 
cut  each  other  in  two  points,  diametrically  opposite,  and 
consequently  their  points  of  section  are  180  degrees 

*  It  is  strongly  recommended  that  young  learners  be  taught  to  verify  the  de- 
finitions in  the  manner  here  proposed. 

grees  is  every  circle  divided  1  Does  the  arc  of  a  small  circle  contain 
the  same  number  of  degrees  as  the  corresppnding  arc  of  a  large 
circle  1  How  many  degrees  in  a  quadrant — in  a  semi-circle  1 

193.  What  figure  does  any  section  of  a  sphere  produce  1    Define 

great  circles— small  circles.     How  may  this  distinction  be  exempli-  • 

lied  1    Define  the  axis  of  a  circle — the  rjole.     How  many  poles  has 

every  great  circle  1  How  many  degrees  is  the  pole  from  the  circum- 

16* 


186  ASTRONOMY. 

apart.  Thus,  if  we  cut  the  apple  through  the  center, 
in  two  different  directions,  we  shall  find  that  the  points 
where  the  circles  intersect  one  another,  are  directly 
opposite  to  each  other,  and  hence  the  distance  between 
them  is  half  round  the  apple,  and,  of  course,  180  de- 
grees. A  point  on  the  sphere,  90  degrees  distant  from 
any  great  circle,  is  the  pole  of  that  circle  ;  and  every 
circle  on  the  globe,  drawn  from  the  pole  to  the  circum- 
ference of  any  circle,  is  at  right  angles  to  it.  Such  a 
circle  is  called  a  secondary  of  the  circle  through  whose 
pole  it  passes. 

194.  In  order  to  fix  the  position  of  any  place,  either 
on  the  surface  of  the  earth  or  in  the  heavens,  both  the 
earth  and  the  heavens  are  conceived  to  be  divided  into 
separate  portions,  by  circles  which  are  imagined  to  cut 
through  them  in  various  ways.  The  earth,  thus 
intersected,  is  called  the  terrestrial,  and  the  heavens 
the  celestial  sphere.  The  great  circles  described  on 
the  earth,  extended  to  meet  the  concave  sphere  of  the 
heavens,  become  circles  of  the  celestial  sphere. 

The  Horizon  is  the  great  circle  which  divides  the 
earth  into  upper  and  lower  hemispheres,  and  separates 
the  visible  heavens  from  the  invisible.  This  is  the 
rational  horizon  :  the  sensible  horizon  is  a  circle  touch- 
ing the  earth  at  the  place  of  the  spectator,  and  is 
bounded  by  the  line  in  which  the  earth  and  sky  seem 
to  meet.  The  poles  of  the  horizon  are  the  zenith  and 
nadir.  The  zenith  is  the  point  directly  over  our  heads; 
the  nadir,  that  directly  under  our  feet.  The  plumb- 
line,  (such  as  is  formed  by  suspending  a  bullet  by  a 
string,)  coincides  with  the  axis  of  the  horizon,  and 
consequently  is  directed  toward  its  poles.  Every 

ference  1  How  does  a  great  circle  passing  through  the  pole  of  another 
great  circle  cut  the  circle  1  What  is  such  a  circle  called  1 

194.  How  are  the  earth  and  heavens  conceived  to  be  divided  1 
What  is  the  terrestrial,  and  what  the  celestial  sphere  1  '  How  do 
terrestrial  circles  become  celestial  *  Define  the  horizon.  Distinguish 
between  the  rational  and  the  sensible  horizon.  Define  the  zenith 


DOCTRINE   OF   THE    SPHERE.  187 

place  on  the  surface  of  the  earth  has  its  own  horizon  ; 
and  the  traveller  has  a  new  horizon  at  every  step, 
always  extending  90  degrees  from  him  in  every 
direction. 

195.  Vertical  circles  are  those  which  pass  through 
the  poles  of  the  horizon,  (the  zenith  and  nadir,)  perpen- 
dicular to  it.      The  Meridian  is  that  vertical  circle 
which  passes  through  the  north  and  south  points.    The 
Prime    Vertical,   is  that  vertical  circle  which  passes 
through  the  east  and  west  points.     The  altitude  of  a 
heavenly  body,    is   its   elevation  above  the   horizon, 
measured  on  a  vertical  circle  ;  the  azimuth  of  a  body 
is  its  distance,  measured  on  the  horizon,  from  the  meri- 
dian to  a  vertical  circle  passing  through  that  body ;  and 
the  amplitude  of  a  body  is  its  distance,  on  the  horizon, 
north  or  south  of  the  prime  vertical. 

196.  In  order  to  make  these  definitions  intelligible 
and  familiar,  I  invite  the  young  learner,  who  is  anxious 
to  acquire  clear  ideas  in  astronomy,  to  accompany  me 
some  fine  evening  under  the  open  sky,  where  we  can 
have  an  unobstructed  view  in  all  directions.     A  ship 
at  sea  would  afford  the  best  view  for  our  purpose,  but 
•a  level  plain  of  great  extent  will  do  very  well.     We 
carry  the  eye  all  round  the  line  in  which  the  sky  seems 
to  rest  upon  the  earth  :  this  is  the  horizon.     I  hold  a 
line  with  a  bullet  suspended,  and  this  shows  me  the 
true  direction  of  the  axis  of  the  horizon  ;  and  I  look 
upward   in   the  direction  of  this   line  to  the  zenith, 
directly  over   my   head,   and   downward   toward   the 
|  nadir.     If  I  mark  the  position  of  a  star  exactly  in  the 

zenith,  as  indicated  by  the  position  of  the  plumb-line, 

and  nadir.  Toward  what  points  is  the  plumb  line  directed  1  How 
many  horizons  can  be  imagined  1 

195.  Define  vertical  circles— the  meridian — the  prime  vertical — 
'  altitude — azimuth — amplitude. 

196.  What  is  proposed  in  order  to  make  these  definitions  intelli- 

e'.ble  and  iamiliarl    What  situation  would  afford  the  best  view  1 
escribe  how  we  shall  successively  denote  the  position  of  the  axis  01 


188  ASTRONOMY. 

and  then  turn  round  and  look  upward  toward  the  zenith,  j, 
I  shall  probably  not  see  the  star,  because  I  do  not  look; 
high  enough.     Most  people  will  find,  if  they  first  fix* 
upon  a  star  as  being  in  the  zenith  when  their  faces  are 
toward  the  south,  and  then  turning  round  to  the  north, 
fix    upon  another  star  as   near  the   zenith,   (without! 
reference  to  the  first,)  they  will  find  that  the  two  stars, 
are  several  degrees  apart,  the  true  zenith  being  half 
way  between  them.     This  arises  from  the  difficulty  of 
looking  directly  upward. 

197.  Having  fixed  upon  the  position  of  the  zenith,  Ii 
will  point  my  finger  to  it,  and  carry  the  finger  down  to: 
the  horizon,  repeating  the  operation  a  number  of  times,; 
from  the  zenith  to  different  points  of  the  horizon  :  the; 
arcs  which  my  finger  may  be  conceived  to  trace  oul 
on  the  face  of  the  sky,  are  "arcs  of  vertical  circles.  IJ 
will  now  direct  my  finger  toward  the  north  point  ow 
the  horizon,  (having  previously  ascertained  its  position.; 
by  a  compass,)  and  carry  it  upward  through  the| 
zenith,  and  down  to  the  south  point  of  the  horizon  : 
this  is  the  meridian.  From  the  south  point,  I  carry 4 
my  finger  along  the  horizon,  first  toward  the  east, 
and  then  toward  the  west,  and  I  measure  off  arcs  of 
azimuth.  I  might  do  the  same  from  the  north  point, 
for  azimuth  is  reckoned  east  and  west  from  either  the* 
north  or  the  south  point.  I  will  again  direct  my, 
finger  to  the  western  point  of  the  horizon,  and  carry, 
it  upward  through  the  zenith  to  the  east  point,  and 
I  shall  trace  out  the  prime  vertical.  From  this, 
either  on  the  eastern  or  the  western  side,  if  Ii 
carry  my  finger  along  the  horizon,  north  and  south, 
I  shall  trace  out  arcs  of  amplitude.  I  will  finally  fix 


the  horizon — the  zenith  and  nadir.    Difficulty  of  looking  directly  to 
the  zenith. 

197.  How  to  mark  out  with  the  ringer  vertical  circles — the  meri- 
dian— arcs  of  azimuth — the  prime  vertical — arcs  of  amplitude — arcs 
of  altitude  1 


DOCTRINE  OF  THE  SPHERE.  189 

my  eve  on  a  certain  bright  star,  and  try  to  determine 
hoV  far  it  is  above  the  horizon.  This  will  be  its  alti- 
I  tilde.  It  appears  to  be  about  one  third  of  the  way 
'from  the  horizon  to  the  zenith  ;  then  its  altitude  is  30 
|  degrees.  But  we  are  apt  to  estimate  the  number  of 
!  degrees  near  the  horizon  too  large,  and  near  the  zenith 
(too  small,  and  therefore  I  look  again  more  attentively, 
:  making  some  allowance  for  this  source  of  error,  and 
I  judge  the  altitude  of  the  star  to  be  about  27  degrees, 
and  of  course  its  zenith  distance  63  degrees. 

19&.  The  Axis  of  the  earth,  is  the  diameter  on  which 
the  earth  is  conceived  to  turn  in  its  daily  revolution 
ifrom  west  to  east.     The  same  line  continued  until  it 
|  meets  the  concave  of  the  heavens,  constitutes  the  axis 
I  of  the  celestial  sphere.     We  will  take  a  large  round 
;  apple,  and  run  a  knitting-needle  through  it  in  the  di- 
jrection  of  the  eye  and  stem.     The  part  of  this  that 
'lies  within  the  apple,  represents  the  axis  of  the  earth, 
:and  its  prolongation  (conceived  to  be  continued  to  the 
]sky,)  the  axis  of  the  heavens.     We  do  not  suppose 
that  there  is  any  such  actual  line  on  which  the  earth 
1  turns,  any  more  than  there  is  in  a  top  on  which  it 
spins ;  but  it  is  nevertheless   convenient   to   imagine 
^such  a  line,  and  to  represent  it  by  a  wire.*     The  poles 
of  the  earth  are  the  extremities   of  the  earth's  axis; 
the  poles  of  the  heavens  are  the  extremities  of  the  ce- 
lestial axis. 

199.  The  Equator  is  a  great  circle,  cutting  the  axis 

of  the  earth  at  right  angles.     The  intersection  of  the 

\  plane  of  the  equator  with  the  surface  of  the  earth,  con- 

1     *  Experience  shows  that  it  is  necessary  to  guard  young  learners  from  the 
j  error  of  supposing  that  our  artificial  representations  of  the  sphere  actually 
represent  things  as  they  are  in  nature. 


193.  Define  the  axis  of  the  earth— axis  of  the  celestial  sphere. 
'Ho\v  are  both  represented  by  means  of  an  apple  7  Is  there  any 
'such  actual  line  on  which  the  earth  turns  1  Distinguish  between  the 
i  poles  of  the  earth  and  the  poles  of  the  heavens. 


190  ASTRONOMY. 

stitutes  the  terrestrial,  and  its  intersection  with  the 
concave  sphere  of  the  heavens,  the  celest.ial  equator. 
We  have  before  seen  (Art.  195)  that  every  place  on  ther 
earth  has  its  own  horizon.  Wherever  one  stands  on  i 
the  earth,  he  seems  to  be  in  the  center  of  a  circle 
which  bounds  his  view.  If  he  is  at  the  equator,  this 
circle  passes  through  both  the  poles ;  or,  in  other- 
words,  at  the  equator  the  poles  lie  in  the  horizon. . 
Let  us  imagine  ourselves  standing  there  on  the  21sti 
of  March,  when  the  sun  rises  due  east  and  sets  due| 
west,  and  appears  to  move  all  day  in  the  celestial! 
equator,  and  let  us  think  how  it  would  seem  to  see  theft1 
sun,  at  noon,  directly  over  our  heads,  and  at  night  tdf 
see  the  north  star  just  glimmering  on  the  north  poinfii 
of  the  horizon.  If  we  sail  northward  from  the  equa-' 
tor,  the  north  star  rises  just  as  many  degrees  above  the| 
horizon  as  we  depart  from  the  equator ;  so  that  by  the 
time  we  reach  the  part  of  the  globe  where  we  live! 
the  north  star  has  risen  almost  half  way  to  the  zenith," 
and  the  axis  of  the  sphere  which  points  toward  theo 
north  star,  seems  to  have  changed  its  place  as  we  have-1 
changed  ours,  and  to  have  risen  up  so  as  to  make  a  i 
large  angle  with  the  horizon,  and  the  sun  no  longer  \ 
mounts  to  the  zenith  at  noon. 

200.  Now  it  is  not  the  earth  that  has  shifted  its  po4 
sition ;  this  constantly  maintains  the  same  place,  and 
so  does  the  equator  and  the  earth's  axis.     Our  horizon 
it  is  that  has  changed ;  as  we  left  the  equator,  a  new 
horizon  succeeded  at  every  step,  reaching  constantly.' 
farther  and   farther  beyond  the  pole  of  the  earth,  on 
dipping  .constantly  more  and  more  below  the  celestial 
pole  ;  but  being  insensible  of  this  change  in  our  hori- 

199.  Define  the  equator.     Distinction  between  the  terrestrial  and 
the  celestial  equator.     Where  do  the  poles  of  the  equator  lie  1    How  * 
would  the  sun  appear  to  move   to   a  spectator   on   the   equator  1 
Where  would  the  north  star  appear  1    How,  when  we  sail  northward 
from  the  equator  1    What  apparent  change  in  the  earth's  axis  7 

200.  What  has  caused  these  changes  1  It' we  sail  quite  to  the  north 


DOCTRINE  OF  THE  SPHERE.  191 

zon,  the  pole  it  is  that  seems  to  rise,  and  if  we  were 
]to  sail  quite  to  the  north  pole  of  the  earth,  the  north 
star  would  be  directly  over  our  heads,  and  the  equator 
would  have  sunk  quite  down  to  the  horizon ;  and  now 
;the  sun,  instead  of  mounting  up  to  the  zenith  at  noon, 
just  skims  along  the  horizon  all  day  ;  and,  at  night,  at 
seasons  of  the  year  when  the  sun  is  south  of  the  equator, 
all  the  stars  appear  to  revolve  in  circles  parallel  to  the 
horizon,  the  circles  of  revolution  continually  growing 
less  as  we  look  higher  and  higher,  until  those  stars 
which  are  near  the  zenith  scarcely  appear  to  revolve 
!at  all.  Those  who  sail  from  the  equator  toward  the 
:pole,  and  see  the  apparent  paths  of  the  sun  and  stars 
•change  so  much,  can  hardly  help  believing  that  those 
! bodies  have  been  changing  their  courses;  but  all  these 
I  appearances  arise  merely  from  the  spectator's  chang- 
jing  his  own  horizon,  that  is,  constantly  having  new 
lones,  which  cut  the  axis  of  the  earth  at  different  an- 
jgles. 

201.  The  Latitude  of  a  place  on  the  earth,  is  its  dis- 
tance from  the  equator,  north  or  south.     The  Longi- 
tude  of  a   place   is   its   distance  from  some  standard 
meridian,  east  or  west.     The  meridian  usually  taken 

^as  the  standard,  is  that  of  the  observatory  of  Green- 
wich, near  London;  and  when  we  say  that  the  longi- 
j  tude  of  New  York  is  74  degrees,  we  mean  that  the 
\  meridian  of  New  York  cuts  the  equator  74  degrees 
west  of  the  point  where  the  meridian  of  Greenwich 
cuts  it. 

202.  The  Ecliptic  is  the  great  circle  in  which  the 
^  earth  performs  its  annual  revolution  around  the  sun. 
;  It  passes  through  the  center  of  the  earth  and  the  cen- 
ter of  the  sun.     It  is  found,  by  observation,  that  the 
earth  does  not  lie  with  its  axis  perpendicular  to  the 

i  pole,  where  will  the  north  star  appear'?  Where  the  equator  1  How 
j  would  the  sun  and  stars  appear  to  revolve  in  their  daily  progress  1 

201.  Define  the  latitude  of  a  place  on  the  earth — the  longitude— 
i  from  what  place  is  it  reckoned  1 


192  ASTRONOMY. 

plane  of  the  ecliptic,  so  as  to  make  the  equator  coin-| 
cide  with  it,  but  that  it  is  turned  about  23£  degreed 
out  of  a  perpendicular  direction,  making  an  angle  with? 
the  plane  itself  of  68|  degrees.  The  equator,  there.] 
fore,  must  be  turned  the  same  distance  out  of  a  coin- 
cidence  with  the  ecliptic,  the  two  circles  making  an 
angle  with  each  other  of  23J  degrees.  The  Equinoc* 
Hal  Points,  or  Equinoxes,  are  the  points  where  the 
ecliptic  and  equator  cross  each  other.  The  time  when 
the  sun  crosses  the  equator  in  going  northward,  is< 
called  the  vernal,  and  in  returning  southward,  the  aw*> 
tumnal  equinox.  The  vernal  equinox  occurs  about! 
the  21st  of  March,  and  the  autumal  about  the  22d  off 
September.  The  Solstitial  Points  are  the  two  point** 
of  the  ecliptic  most  distant  from  the  equator.  ThJ 
times  when  the  sun  comes  to  them  are  called  the  SolA 
stices.  The  summer  solstice  occurs  about  the  22<M 
of  June,  and  the  winter  solstice  about  the  22d  of  De4 
cember* 

203.  The  ecliptic  is  divided  into  twelve  equal  parts, 
of  30  degrees  each,  called  Signs,  which,  beginning  at 
the  vernal  equinox,  succeed  each  other  in  the  follow-' 
ing  order,  being  each  distinguished  by  characters  or'i 
symbols,  by  which  the  student  should  be  able  to  re-| 
cognise  the  signs  to  which  they  severally  belong  whence- 
ever  he  meets  with  them. 


1.  Aries, 

T 

7.  Libra,                ^ 

2.  Taurus, 

« 

8.   Scorpio,             1H. 

3.  Gemini, 

n 

9.  Sagittarius,        1 

4.  Cancer, 

£3 

10.  Capricornus,    Y? 

5.  Leo, 

SI 

11.  Aquarius,         2£ 

6.  Virgo, 

1* 

12.  Pisces,              >£ 

202.  Define  the  ecliptic.    What  is  the  angle  of  inclination  of  the 
ecliptic  to  the  equator'?    What  are  the  equinoctial  points  or  equi- 
noxes— the  vernal  equinox — the  autumnal — the  solstitial   points — 
the  solstices  1    When  do  they  occur  1 

203.  Iio\v  is  the  ecliptic  divided  1    Name  the  signs  of  the  zodiac 
and  recognise  each  by  its  character. 


DOCTRINE   OF  THE   SPHERE.  193 

204.  The  position  of  a  heavenly  body  is  referred  to 
by  its  right  ascension  and  declination,  as  in  Geography 
we  determine  the  situation  of  places  by  their  latitudes 
and  longitudes.     Right  Ascension  is  the  angular  dis- 
tance from  the  vernal  equinox,  reckoned  on  the  celes- 
tial equator,  as  we  reckon  longitude  on  the  terrestrial 
equator  from  Greenwich.     Declination  is  the  distance 
of  a  body  from  the  celestial  equator,  either  north  or 
south,  as  latitude  is  counted  from  the  terrestrial  equa- 
tor.    Celestial  Longitude  is  reckoned  on  the  ecliptic 
from  the  vernal  equinox,  and  celestial  Latitude  from 
the  ecliptic,  north  or  south. 

205.  Parallels  of  Latitude  are  small  circles  parallel 
jto  the  equator.     They  constantly  diminish  in  size,  as 
we  go  from  the  equator  to  the  pole.     The  Tropics  are 
the  parallels  of  latitude  which  pass  through  the  sol- 
Istices.     The  northern  tropic  is  called  the  tropic  of  Can- 
cer ;  the  southern,  the  tropic  of  Capricorn.     The  Po- 
lar Circles  are  the  parallels  of  latitude  that  pass  through 
the  poles  of  the  ecliptic,  23J  degrees  from  the  poles  of 
the  earth.     That  portion  of  the  earth  which  lies  be- 
tween the  tropics,  on  either  side  of  the  equator,  is  called 
the  Torrid  Zone ;    that  between  the  tropics  and  the 
-polar  circles,  the  Temperate  Zone ;  and  that  between 
the  polar  circles  and  the  poles,  the  Frigid  Zone.     The 
Zodiac  is  the  part  of  the  celestial  sphere  which  lies 
about  eight  degrees  on  each  side  of  the  ecliptic.     This 
portion  of  the  heavens  is  thus  marked  off  by  itself  be- 
cause the  paths  of  the  planets  are  confined  to  it. 

206.  After  having  endeavored  to  form  the  best  idea 
,we  can  of  the  circles,  and  of  the  foregoing  definitions 
[relating  to  the  sphere,  we  shall  derive  much  aid  from 


204.  Define  right    ascension — declination — celestial   longitude — 
celestial  latitude. 

205.  Parallels  of  latitude— how  do  they  change  as  we  go  from 
the  equator'?    The  tropics — polar  circles— torrid  zone— -temperate 
^one — frigid  zones — zodiac. 

17 


194  ASTRONOMY. 

inspecting  an  artificial  globe,   and   seeing  how  these  •[ 
various  particulars  are  represented  there.     But  every  i 
learner,  however  young,  can  adopt,  with  great  advan- 1 
tage,  the  following  easy  device  for  himself.     To  repre-  \ 
sent  the  earth,  select  a  large  apple,  (a  melon,  when  in-'; 
season,  will  be  found  still  better.)     The  eye  and  the 
stem  of  the  apple  will  indicate  the  position  of  the  two  j 
poles  of  the  earth.     Applying  the  thumb  and  finger  | 
of  the  left  hand  to  the  poles,  and  holding  the  apple  so  I 
that  the  poles  may  be  in  a  north  and  south  line,  turaj 
this  little  globe  from  west  to  east,  and  its  motion  will ; 
correspond  to  the  daily  motion  of  the  earth.     Pass  a 
wire  or  a  knitting-needle  through  the  poles,  and  it  wiltl 
represent  the  axis  of  the  sphere.     A   circle  cut  round  \ 
the  apple  half  way  between  the  poles,  will  be  the  \ 
equator ;    and  several  other  circles  cut  between   the  \ 
equator  and  the  poles,  parallel  to  the  equator,  will  re- 
present 'parallels  of  latitude  ;  of  which  two,  drawn  23£  ! 
degrees  from  the  equator,  will   be  the  tropics,  and  two  j 
others,  at  the  same  distance  from  the  poles,  will  be  the  ! 
polar  circles.     The  space  between  the  tropics,  on  both  j 
sides  of  the  equator,  will  be  the  torrid  zone  ;  between 
the  tropics  and  polar  circles,  the  two  temperate  zones  ; 
and  between  the  polar  circles  and  the  poles,  the  twcl 
frigid  zones.     A  great   circle   cut   round   the  apple,  I 
passing  through  both  poles,  in  a  north  and  south  direc-  J 
tion,  will    represent  the   meridian,  and  several    other  \ 
great  circles  drawn  through  the  poles,  and,  of  course,  , 
perpendicularly  to  the  equator,  will  be  secondaries  to 
the  equator,  constituting  meridians,  or  hour  circles.     A 
great  circle,  cut  through  the  center  of  the  apple,  from 
one  tropic  to  the  other,  would  represent  the  plane  of  the 


206.  After  forming  as  clear  an  idea  as  we  can  of  the  divisions  of 
the  sphere,  to  what  aids  shall  we  resort  1  How  shall  we  represent 
the  earth — its  poles — the  daily  motion — axis—equator — parallels  of 
latitude — tropics — polar  circles — zones— meridians  or  hour  circles — 
solstices— equinoctial  points'? 


INSTRUMENTS   AND    OBSERVATIONS.  195 

J  ecliptic,  and  its  intersection  with  the  surface  of  the 
|  apple,  would  be  the  terrestrial  ecliptic.  The  points 
where  this  circle  meets  the  tropics,  indicate  the  position 
of  the  solstices ;  and  its  intersections  with  the  equator, 
the  equinoctial  points. 


CHAPTER    II. 

ASTRONOMICAL  INSTRUMENTS  AND  OBSERVA- 
TIONS. 

TELESCOPE TRANSIT      INSTRUMENT ASTRONOMICAL     CLOCK SEX- 
TANT. 

207.  WHEREVER  we  are  situated  on  the  surface  of  the 
earth,  we  appear  to  be  in  the  center  of  a  vast  sphere, 
on  the  concave  surface  of  which  all  celestial  objects 
are  inscribed.  If  we  take  any  two  points  on  the  sur- 
face of  the  sphere,  as  two  stars,  for  example,  and 
imagine  straight  lines  to  be  drawn  from  them  to  the  eye, 
the  angle. included  between  these  lines  will  be  measured 
by  the  arc  of  the  sky  contained  between  the  two 
Fig.  97. 


points.     Thus,  if  D  B  H,  Fig.  97,  represents  the  con- 

207.  How  to  measure  the  angular  distance  between  two  stars. 
Illustrate  by  Fig.  97.  Why  may  we  measure  the  angle  on  the  small 
circle  G  F  K 1 


196  ASTRONOMY. 

cave  surface  of  the  sphere,  A,  B,  two  points  on  it,  as 
two  stars,  and  C  A,  C  B,  straight  lines  drawn  from  the 
spectator  to  those  points,  then  the  angular  distance  be- 
tween them  is  measured  by  the  arc  A  B,  or  the  angle 
A  C  B.  But  this  angle  may  be  measured  on  a  much 
smaller  circle,  G  F  K,  since  the  arc  E  F  will  have 
the  same  number  of  degrees  as  the  arc  A  B. 

208.  The  simplest  mode  of  taking  an  angle  between 
two  stars,  is  by  means  of  an  arm  opening  at  the  joint 
like  the  blade  of  a  penknife,  the  end  of  the  arm  moving 
like  C  E  upon  the  graduated  circle  G  F  K.  In  fact, 
an  instrument  constructed  on  this  principle,  resembling 
a  carpenter's  rule  with  a  folding  joint,  with  a  semicircle 
attached,  constituted  the  first  rude  apparatus  for  meas- 
uring the  angular  distance  between  two  points  on  the 
celestial  sphere.  Thus,  the  sun's  elevation  above  the 
horizon  might  be  ascertained  by  placing  one  arm  of 
the  rule  on  a  level  with  the  horizon,  and  bringing  the 
edge  of  the  other  into  a  line  with  the  sun's  centre. 
The  common  surveyor's  compass  affords  a  simple  ex- 
ample of  angular  measurement.  Here  the  needle  lies 
in  a  north  and  south  line,  while  the  circular  rim  of  the 
compass,  when  the  instrument  is  level,  corresponds  to 
the  horizon.  Hence,  the  compass  shows  the  azimuth 
of  an  object,  or  how  many  degrees  it  is  east  or  west 
of  the  meridian.  In  several  astronomical  instruments, 
the  telescope  and  graduated  circles  are  united ;  the 
telescope  enables  us  to  see  minute  objects  or  points, 
and  the  graduated  circle  enables  us  to  measure  angu- 
lar distances  from  one  point  to  another.  The  most  im- 
portant astronomical  instruments  are  the  Telescope,  the 
Transit  Instrument,  the  Astronomical  Clock,  and  the 
Sextant. 

203.  What  is  the  simplest  mode  of  taking  the  angle  between  two 
stars  1  Example  of  angular  measurement  l>y  the  surveyor's  com- 
pass. What  angle  or  arc  does  it  measure  1  Why  do  some  instru- 
ments unite  the  telescope  with  a  graduated  circle  1 


INSTRUMENTS  AND  OBSERVATIONS.  197 

209.  The  Telescope  has  been  already  described  and 
its  principles  explained,  (Art.  184.)     We  have  seen 
that  it  aids  the  eye  in  two  ways  :  first,  by  collecting 
and  conveying  to  the  eye  a  larger  beam  of  light  than 
would  otherwise  enter  it,  thus  rendering  objects  more 
distinct,  and  many  visible  that  would  otherwise  be  in- 
visible for  want  of  sufficient  light ;  and,  secondly,  by 
enlarging  the  angle  under  which  objects  are  seen,  and 
thus  bringing  distinctly  into  view  such  as  are  invisible, 
or  obscure  to  the  naked  eye  from  their  minuteness. 
When  the  telescope  is  used  by  itself,  it  is  for  obtain- 
ing brighter  and  more  enlarged  views  of  the  heavenly 
bodies,  especially  the  moon  and  planets.     With   the 
larger  kinds  of  telescopes,  we  obtain  many  grand  and 
interesting  views  of  the  heavens,  and  see  millions  of 
worlds  revealed  to  us  that  are  invisible  to  the  naked 
eye. 

210.  The  Transit  Instrument  (Fig.  98,  p.  198)  is  a 
telescope  firmly  fixed  on  a  stand,  so  as  to  keep  it  per- 
fectly steady,  and  permanently  placed  in  the  meridian. 
The  object  of  it  is  to  determine  when  bodies  cross  the 
meridian,  or  make  their  transit  over  it ;  or,  in  other 
words,  to  show  the  precise  instant  when  the  center  of 
a  heavenly  body  is  on  the  meridian.     The  Astronomi- 
cal Clock  is  the  constant  companion  of  the  transit  in- 
strument.    This  clock  is  so  regulated  as  to  keep  exact 
pace  with  the  stars,  which  appear  to  move  round  the 
earth  from  east  to  west  once  in  twenty-four  hours,  in 
consequence  of  the  earth  turning  on  its  axis  in  the 
same  time  from  west  to  east.     The  time  occupied  in 
one  complete  revolution  of  the  earth,  (which  is  indica- 
ted by  the  interval  occupied  by  a  star  from  the  me- 

209.  How  does  the  telescope  aid  the  eye  1    When  the  telescope  is 
used  by  itself,  for  what  purpose  is  if?    What  views  do  we  obtain 
with  the  larger  kinds  of  telescopes! 

210.  Define  the  Transit  Instrument.    What  is  the  object  of  it  * 
What  does  it  show  1    What  instrument  accompanies  it  1    With 
what  does  the  astronomical  clock  keep  pace  1    What  occasions  the 

17* 


198 


ASTRONOMY. 


ridian  round  to  the  meridian  again,)  is  called  a  sidereal 
day.     It  is,  as  we  shall  see  hereafter,  shorter  than  the 

Fig.  98. 


solar  day  as  measured  by  the  return  of  the  sun  to  the 
meridian.  The  astronomical  clock  is  so  regulated  as 
to  measure  the  progress  of  a  star,  indicating  an  hour 
for  every  fifteen  degrees,  and  twenty-four  hours  for  the 
whole  period  of  the  revolution  of  a  star.  Sidereal  time 
commences  when  the  vernal  equinox  is  on  the  meridian, 
just  as  solar  time  commences  when  the  sun  is  on  the 
meridian. 


apparent  movement  of  the  stars  from  east  to  west  1  Define  a  side-* 
real  day.  To  how  many  degrees  does  an  hour  correspond  1  Wheu 
does  sidereal  time  commence  1 


INSTRUMENTS  AND  OBSERVATIONS.  199 

211.  Any  thing  becomes  a  measure  of  time  which 
-divides  duration  equally.  The  celestial  equator,  there- 
fore, is  precisely  adapted  to  this  purpose,  since,  in  the 
-daily  revolution  of  the  heavens,  equal  portions  of  it 
pass  under  the  meridian  in  equal  times.  The  only 
difficulty  is,  to  ascertain  the  amount  of  these  portions 
for  given  intervals.  Now  the  astronomical  clock 
shows  us  exactly  this  amount,  for,  when  regulated  to 
.sidereal  time,  the  hour  hand  keeps  exact  pace  with. the 
vernal  equinox,  revolving  once  on  the  dial  plate  of  the 
.clock  while  the  equator  turns  once  by  the  revolution 
.of  the  earth.  The  same  is  true,  also,  of  all  the  small 
circles  of  diurnal  revolution  :  they  all  turn  exactly  at 
the  same  rate  as  the  equator,  and  a  star  situated  any- 
where between  the  equator  and  the  pole,  will  move  in 
its  diurnal  circle  along  with  the  clock,  in  the  same 
manner  as  though  it  were  in  the  equator.  Hence,  if 
live  note  the  interval  of  time  between  the  passage  of 
#ny  two  stars,  as  shown  by  the  clock,  we  have  a  meas- 
ure of  the  number  of  degrees  by  which  they  are  distant 
from  each  other  in  right  ascension.  We  see  now  how 
.easy  it  is  to  take  arcs  of  right  ascension  :  the  transit 
instrument  shows  us  when  a  body  is  on  the  meridian  ; 
the  clock  indicates  how  long  it  is  since  the  vernal  equi- 
nox passed  it,  which  is  the  right  ascension  itself.  (Art. 
•204.)  It  also  tells  us  the  difference  of  right  ascension 
between  any  two  bodies,  simply  by  indicating  the  dif- 
ference in  time  between  their  periods  of  passing  the 
meridian.  I  observed  a  star  pass  the  central  wire  of 
the  transit  instrument  (which  was  exactly  in  the  me- 
ridian) three  hours  and  fifteen  minutes  of  sidereal  time  ; 
hence,  as  one  hour  equals  fifteen  degrees,  three  hours 

211.  How  may  any  thing  become  a  measure  of  time  1  Why  is  the 
celestial  equator  peculiarly  adapted  to  this  purpose  1  What  is  the 
only  difficulty  1  How  does  the  astronomical  clock  show  us  what  por- 
tion of  the  equator  passes  under  the  meridian'?  Do  the  parallels  of 
latitude  turn  at  the  same  rate  with  the  equator  1  How  do  we  meas- 
ure the  difference  of  right  ascension  between  two  stars,  by  means 


200  ASTRONOMY. 

and  a  quarter  must  have  equalled  forty-eight  degrees 
and  three  quarters,  which  was  the  right  ascension  of 
the  star.  Two  hours  and  three  quarters  afterward, 
that  is,  at  six  hours  sidereal  time,  I  observed  another 
star  cross  the  meridian.  Its  right  ascension  must  have 
been  ninety  degrees,  and  consequently  the  difference 
of  right  ascension  of  the  two,  forty-one  and  a  quarter 
degrees. 

212.  Again,  it  is  easy  to  take  the  declination  of  a 
body  when  on  the  meridian.     By  declination,  we  must 
recollect,  is  meant  the  distance  of  a  body  north  or  south 
of  the  celestial  equator.     When  a  star  is  crossing  the 
meridian  line  of  the  transit  instrument,  the  point  of  the 
meridian  toward  which  the  telescope   is  directed    at 
that  instant,  will  be  shown  on  the  graduated  circle  of 
the  instrument,  and  the  distance  of  that  point  from  the 
zenith,  subtracted  from  the  latitude  of  the  place  of  ob- 
servation, will  give  the  decimation  of  the  star.     We 
have  before  seen,  that  when  we  have  found  the  right 
ascensions  and  declinations  of  the  heavenly  bodies,  we 
may  lay  down  their  relative  situations  on  a  map,  just 
as  we  do  those  of  places  on  the  earth  by  their  latitudes 
and  longitudes. 

213.  The  Sextant  is  an  instrument  used  for  taking 
the  angular  distance  of  one  point  from  another  on  the 
celestial  sphere.     It  is  particularly  valuable  for  meas- 
uring celestial  arcs  at  sea,  because  it  is  not,  like  most 
astronomical  instruments,  affected  by  the  motion  of  the 
ship.     The  principle  of  the  sextant  may  be  briefly 
described  as  follows  :  it  gives  the  angular  distance  be- 
tween any  two  objects  on  the  celestial  sphere,  by  re- 
flecting the  image  of  one  of  the  objects  so  as  to  coin- 


of  the  ctock'?    Describe  the  mode  of  taking  right  ascensions  with 
the  transit  instrument  and  clock. 

212.  What  is  the  declination  of  a  body  1    How  taken  when  on 
the  meridian  1 

213.  For  what  is  the  Sextant  used  1    For  what  is  it  particularly 


INSTRUMENTS  AND  OBSERVATIONS. 


201 


cide  with  the  other  object  as  seen  by  the  naked  eye. 
The  arc  through  which  the  reflector  is  turned  to  bring 
the  reflected  object  to  coincide  with  the  other  object, 
becomes  a  measure  of  the  angular  distance  between 
them.  The  instrument  is  of  a  triangular  shape,  and 

Fig.  99. 


is  made  strong  and  firm  by  metallic  cross-bars.     It 
i  has  two  small  mirrors,  I,  H,  called  respectively,  the 
I  index  glass  and  the  horizon  glass,  both  of  which  are 
firmly  fixed  perpendicularly  to  the  plane  of  the  in- 
strument.    The  index  glass  is  attached  to  the  movable 
arm,  I  D,  and  turns  as  this  is  moved  along  the  gradu- 

i  valuable  1    State  its  principle.     Describe  the  Sextant.    Point  out 
|  the  Index  glass  and  the  Horizon  glass.    State  the  use  of  the  Vernier. 


202  ASTRONOMY. 

ated  limb,  E  F.  This  arm  carries  a  Vernier  at  D,  a 
contrivance  which  enables  us  to  read  off  minute  parts 
of  the  spaces  into  which  the  limb  is  divided.  The 
horizon  glass,  H,  consists  of  two  parts;  the  upper  part 
being  transparent  or  open,  so  that  the  eye  looking 
through  the  telescope,  T,  can  see  through  it  a  distant 
object,  as  a  star,  at  S,  while  the  lower  part  is  a  reflect- 
or. Suppose  it  were  required  to  measure  the  dis- 
tance between  the  moon  and  a  certain  star,  the  moon 
being  at  M,  and  the  star  at  S.  The  instrument  is  held 
firmly  in  the  hand,  so  that  the  eye,  looking -through 
the  telescope,  sees  the  star,  S,  through  the  transparent 
part  of  the  horizon  glass.  Then  the  movable  arm, 
I  D,  is  moved  from  F  toward  E,  until  the  image  of  M 
is  reflected  down  to  S ;  when  the  number  of  degrees 
and  parts  of  a  degree  reckoned  on  the  limb  from  F  to 
the  index  at  D,  will  show  the  angular  distance  be- 
tween the  two  bodies.  The  altitude  of  the  sun  above 
the  horizon,  at  any  time,  may  be  taken  by  looking  di- 
rectly at  the  line  of  the  horizon  (which  is  well  defined 
at  sea)  and  moving  the  index  from  F  toward  E,  until 
the  limb  of  the  sun  just  grazes  the  horizon. 


CHAPTER   III. 
TIME.    PARALLAX.    REFRACTION.     TWILIGHT. 

6IDEREAL  AND  SOLAR  DAYS — MEAN  AND  APPARENT   TIME — HORIZON- 
TAL PARALLAX LENGTH  OF  TWILIGHT  IN  DIFFERENT  COUNTRIES. 

214.  As  time  is  a  measured  portion  of  indefinite  du- 
ration, any  thing  or  any  event  which  takes  place  at 
equal  intervals,  may  become  a  measure  of  time.  But 
the  great  standard  of  time  is  the  period  of  the  revolu- 


Describe  the  Horizon  glass.    Describe  the  mode  of  taking  an  ol>» 
eervation  with  the  Sextant.    How  to  take  the  sun's  altitude. 


TIME.  203 

tion  of  the  earth  on  its  axis,  which,  by  the  most  exact 
observations,  is  found  to  be  always  the  same.  The 
time  of  the  earth's  revolution  on  its  axis,  as  already 
explained,  is  called  a  sidereal  day,  and  is  determined 
by  the  apparent  revolution  of  a  star  in  the  heavens. 
This  interval  is  divided  into  twenty-four  sidereal  hours. 
215.  Solar  time  is  reckoned  by  the  apparent  revolu- 
tion of  the  sun  from  noon  to  noon,  that  is,  from  the 
meridian  round  to  the  meridian  again.  Were  the  sun 
stationary  in  the  heavens  like  a  fixed  star,  the  time  of 
its  apparent  revolution  would  be  equal  to  the  revolu- 
tion of  the  earth  on  its  axis,  and  the  solar  and  sidereal 
days  would  be  equal.  But  since  the  sun  passes  from 
west  to  east,  in  his  apparent  annual  revolution  around 
the  earth,  three  hundred  and  sixty  degrees  in  three 
hundred  and  sixty-five  days,  he  moves  eastward  nearly 
a  degree  a  day.  While,  therefore,  the  earth  is  turn- 
ing once  on  its  axis,  the  sun  is  moving  in  the  same  di- 
rection, so  that  when  we  have  come  round  under  the 
same  celestial  meridian  from  which  we  started,  we  do 
not  find  the  sun  there,  but  he  has  moved  eastward 
nearly  a  degree,  and  the  earth  must  perform  so  much 
inore  than  one  complete  revolution,  before  our  meri- 
dian cuts  the  sun  again.  Now,  since  we  move  in  the 
diurnal  revolution,  fifteen  degrees  in  sixty  minutes, 
we  must  pass  over  one  degree  in  four  minutes.  It 
takes,  therefore,  four  minutes  for  us  to  catch  up  with 
the  sun,  after  we  have  made  one  complete  revolution. 
Hence,  the  solar  day  is  almost  four  minutes  longer  than 
the  sidereal ;  and  if  we  were  to  reckon  the  sidereal 
day  twenty-four  hours,  we  should  reckon  the  solar  day 
twenty-four  hours  and  four  minutes.  To  suit  the  pur- 


214.  What  may  become  a  measure  of  time  1    What  is  the  great 
standard  of  time  1 

215.  Distinguish  between  sidereal  and  solar  time.    Why  are  the  so- 
lar days  longer  than  the  sidereal  1    How  much  longer  1    If  we  count 
the  solar  day  twenty-four  hours,  how  long  is  the  sidereal  day  1 


204  ASTRONOMY. 

poses  of  society  at  large,  however,  it  is  found  more' 
convenient  to  reckon  the  solar  day  twenty-four  hours, 
and  throw  the  fraction  into  the  sidereal  day.     Then, 

24h  4m  :  24h  :  :  24h  :  23h  56m  4s. 
That  is,  when  we  reduce  twenty-four  hours  and  four 
minutes  to  twenty-four  hours,  the  same  proportion  will 
require  that  we  reduce  the  sidereal  day  from  twenty- 
four  hours  to  twenty-three  hours  fifty-six  minutes  four 
seconds ;  or,  in  other  words,  a  sidereal  day  is  such  a  ; 
part  of  a  solar  day. 

216.  The  solar  days,  however,  do  not  always  differ 
from  the  sidereal  by  precisely  the  same  fraction,  since 
they  are  not  constantly  of  the  same  length.  Time,  as 
measured  by  the  sun,  is  called  apparent  time,  and  a 
clock  so  regulated  as  always  to  keep  exactly  with  the 
sun,  is  said  to  keep  apparent  time.  But  as  the  sun  in 
his  apparent  motion  round  the  earth  once  a  year,  goes 
sometimes  faster  and  sometimes  slower,  a  clock  which 
always  keeps  with  the  sun  must  vary  its  motion  ac- 
cordingly, making  some  days  longer  than  others.  The 
average  length  of  all  the  solar  days  throughout  the  year, 
constitutes  Mean  Time.  Clocks  and  watches  are  com- 
monly regulated  to  mean  time,  and  therefore  do  not 
keep  exactly  with  the  sun,  but  are  sometimes  faster 
and  sometimes  slower  than  the  sun.  If  one  clock  is  j 
so  constructed  as  to  keep  exactly  with  the  sun,  and" 
another  clock  is  regulated  to  mean  time,  the  difference 
between  the  two  clocks  at  any  period  is  the  equation 
of  time  for  that  period.  The  two  clocks  would  differ 
most  about  the  third  of  November,  when  the  apparent 
time  is  sixteen  and  a  quarter  minutes  faster  than  the 
mean  time.  But  since  apparent  time  is  at  one  time 
greater  and  at  another  less  than  mean  time,  the  two 

216.  Do  the  solar  days  always  dilier  from  the  sidereal  by  the  same 
fraction  1  What  is  apparent  time  1  When  is  a  clock  said  to  keep 
apparent  time  1  What  constitutes  mean  time  1  How  are  clocks  and 
watches  commonly  regulated  1  What  is  the  equation  of  time  1 
WThen  would  the  two  ^clocks  differ  most,  and  how  much  ?  When 
would  they  be  together  1 


TIME.      PARALLAX.  205 

must  obviously  be  sometimes  equal  to  each  other. 
This  is  the  case  four  times  a  year ;  namely,  April  15th, 
June  15th,  September  1st,  and  December  24th. 

217.  As  a  day  is  the  period  of  the  revolution  of  the 
earth  on  its  axis,  so  a  year  is  the  period  of  the  revolu- 
tion of  the  earth  around  the  sun.  This  time,  which 
constitutes  the  astronomical  year,  has  been  ascertained 
with  great  exactness,  and  found  to  be  365d.  5h.  48m. 
51  sec.  The  ancients  omitted  the  fraction,  and  reck- 
oned it  only  365  days.  Their  year,  therefore,  would 
end  about  six  hours  before  the  sun  had  completed  his 
apparent  revolution  in  the  ecliptic,  and,  of  course,  be 
so  much  too  short.  In  four  years  they  would  disagree 
a  whole  day.  This  is  the  reason  why  every  fourth 
year  is  made  to  consist  of  366  days,  by  reckoning  29 
days  in  February  instead  of  28.  This  fourth  year  the 
ancients  called  Bissextile — we  call  it  Leap  year. 
Fig.  100. 
O P 

:> — 

M, 


218.  PARALLAX  is  the  apparent  change  of  place  which 
objects  undergo  by  being  viewed  from  different  points. 

217.  What  period  is  a  year  1  What  is  its  exact  length  7  How 
long  did  the  ancients  reckon  it  1  Explain  why  every  fourth  year 
is  reckoned  366  days. 

IS 


206 


ASTRONOMY. 


All  objects  beyond  a  certain  moderate  height  above 
us,  appear  to  be  projected  on  the  face  of  the  sky ;  but 
spectators  at  some  distance  from  each  other  refer  the 
same  body  to  different  points  of  the  sky.  Thus,  if 
M  N  (Fig.  100)  represents  the  sky,  and  C  and  D  two 
bodies  in  the  atmosphere,  a  spectator  at  A  would  refer 
C  to  M,  while  one  at  B  would  refer  it  to  N.  The 
arc,  M  N,  would  measure  the  angle  of  the  parallax. 
In  the  same  manner,  O  P  would  measure  the  angle 
of  parallax  of  the  body  D.  It  is  evident  from  the  figure, 
that  nearer  objects  have  a  much  greater  parallax  than 
those  that  are  remote.  Indeed,  the  fixed  stars  are  so 
distant,  that  two  spectators  a  hundred  millions  of  miles 
apart  would  refer  a  given  star  to  precisely  the  same 
part  of  the  heavens.  But  the  moon  is  comparatively 
near,  and  her  apparent  place  in  the  sky,  at  a  given 
time,  is  much  affected  by 
parallax.  Thus,  to  a  spec- 
tator at  A,  the  moon  would 
appear  in  the  sky  at  D, 
while  to  one  at  B,  it  would 
appear  at  C.  Hence,  since 
the  same  body  often  appears 
at  the  same  time  differently 
situated  to  spectators  in  dif- 
ferent parts  of  the  earth, 
astronomers  have  agreed  to 
consider  the  true  situation 
of  a  body  to  be  that  where 
it  would  appear  in  the  sky 
if  viewed  from  the  center 
of  the  earth. 


218.  Define  parallax.  Where  do  all  objects  at  a  certain  heieht 
appear  to  be  projected  '1  How  is  the  same  body  projected  by  dif- 
ferent spectators  1  When  have  objects  a  large  and  when  a  small 
parallax  1  What  is  said  of  the  fixed  stars  1  Of  the  moon  1  What 
do  astronomers  consider  the  true  place  of  a  body  1 


REFRACTION.     TWILIGHT.  207 

219.  The  change  of  place  which  a  body  seen  in  the 
horizon,  by  a  spectator  on  the   surface  of  the  earth, 
would  undergo  if  viewed   from  the  center,   is  called 
horizontal  parallax.     Although  we   cannot   go  to  the 
center  of  the  earth  to  view  it,  yet  we  can  determine 
by  the  aid  of  geometry  where  it  would  appear  if  seen 
from  the  center,  and  hence  we  can  find  the  amount  of 
the  horizontal  parallax  of  a  heavenly  body,  as  the  sun 
or  moon.     When  we  know  the  horizontal  parallax  of 
a  heavenly  body,  we  can  ascertain  its  distance  from  us ; 
but  the  method  of  doing  this  cannot  be  clearly  under- 
stood without  some  knowledge  of  trigonometry. 

220.  REFRACTION  is  a   change  of  place   which   the 
heavenly  bodies  seem  to  undergo,  in  consequence  of  the 
direction  of  their  light  being  altered  in  passing  through 
the  atmosphere.     As  a  ray  of  light  traverses  the  atmo- 
sphere, it  is  constantly  bent  more  and  more,  by  the  re- 
fraction of  the  atmosphere,  out  of  its  original  direction. 
Now  an  object  always  appears   in   that  direction  in 
which  the  light  from  it  finally  comes  to  the  eye.     By 
refraction,  therefore,  the  heavenly  bodies  are  all  made 
to  appear  higher  than  they  really  are,  especially  when 
they  are  near  the  horizon.     The  sun  and  moon,  when 
near  rising  or  setting,  are  elevated  by  refraction  more 
than  their  whole  diameter,  so  that  they  appear  above 
the  horizon  both  before  they  have  actually  risen  and 
after  they  have  set. 

221.  TWILIGHT  is  that  illumination  of  the  sky  which 
takes  place  before  sunrise  and  after  sunset,  by  means  of 
which  the  day  advances  and  retires  by  a  gradual  in- 
crease or  diminution  of  the  light.     While  the  sun  is 
within  eighteen  degrees  of  the  horizon,  some  portion 


219.  What  is  horizontal  parallax  1    "What  use  is  made  of  horizon- 
tal parallax  1 

220.  Define  refraction.    How  is  a  ray  of  light  affected  by  travers- 
ing the  atmosphere  1    How  does  refraction  aflect  the  apparent  places 
of  the  heavenly  bodies  1    What  is  said  of  the  sun  and  moon1. 


209  ASTRONOMY. 

of  its  light  is  conveyed  to  us  by  means  of  the  numerous 
reflexions  from  the  atmosphere.  At  the  equator,  where 
the  circles  of  daily  motion  are  perpendicular  to  the 
horizon,  the  sun  descends  through  eighteen  degrees  in 
an  hour  and  twelve  minutes.  In  tropical  countries, 
therefore,  the  light  of  day  rapidly  declines,  and  as 
rapidly  advances  after  daybreak  in  the  morning.  At 
the  pole,  a  constant  twilight  is  enjoyed  while  the  sun 
is  within  eighteen  degrees  of  the  horizon,  occupying 
nearly  two-thirds  of  the  half  year,  when  the  direct 
light  of  the  sun  is  withdrawn,  so  that  the  progress 
from  continual  day  to  constant  night  is  exceedingly 
gradual.  To  an  inhabitant  of  one  of  the  temperate 
zones,  the  twilight  is  longer  in  proportion  as  the  place 
is  nearer  the  elevated  pole. 


CHAPTER   IV. 


THE  SUN. 


DISTANCE MAGNITUDE QUANTITY    OF  MATTER SPOTS NATURE 

AND    CONSTITUTION REVOLUTIONS SEASONS. 

222.  THE  distance  of  the  sun  from  the  earth  is  about 
ninety-five  millions  of  miles.  Although,  by  means 
of  the  sun's  horizontal  parallax,  astronomers  have  been 
able  to  find  this  distance  in  a  way  that  is  entitled  to 
the  fullest  confidence,  yet  such  a  distance  as  95,000,000 
of  miles  seems  almost  incredible.  Still  it  is  but 
small  compared  with  the  distance  of  the  fixed  stars. 
Let  us  make  an  effort  to  form  some  idea  of  this  vast 
distance,  which  we  shall  do  best  by  gradual  approaches 
to  it.  We  will  then  begin  with  so  small  a  distance 

221.  Define  twilight.    How  far  is  the  sun  below  the  horizon  when 
the  twilight  ceases  1    How  is  it  at  the  equator— at  the  poles — and  in 
the  middle  latitudes  1 

222.  Distance  of  the  sun  from  the  earth.  How  does  it  compare  with 


THE   SUN.  209 

as  that  across  the  Atlantic  ocean,  and  follow  in  mind 
a  ship,  as  she  leaves  the  port  of  New  York,  and  after 
twenty  days'  sail  reaches  Liverpool.  Having  formed 
the  best  idea  we  can  of  this  distance,  we  may  then 
reflect,  that  it  would  take  a  ship,  moving  constantly  at 
the  rate  of  ten  miles  an  hour,  more  than  a  thousand 
years  to  reach  the  sun. 

223.  The  diameter  of  the  sun  is  toward  a  million 
of  miles ;  or,  more  exactly,  it  is  885,000  miles.  One 
hundred  and  twelve  bodies  as  large  as  the  earth,  lying 
side  by  side,  would  be  required  to  reach  across  the  solar 
Usk  ;  and  our  ship,  'sailing  at  the  same  rate  as  before, 
would  be  ten  years  in  passing  over  the  same  space. 
Immense  as  is  the  sun,  we  can  readily  understand  why 
it  appears  no  larger  than  it  does,  when  we  reflect  that 
its  distance  is  still  more  vast.  Even  large  objects  on 
the  earth,  when  seen  on  a  distant  eminence,  or  over  a 
wide  expanse  of  waters,  dwindle  almost  to  a  point. 
Could  we  approach  nearer  and  nearer  to  the  suri,  it 
would  constantly  expand  its  volume  until  it  finally  filled 
the  whole  sky.  We  could,  however,  approach  but  little 
nearer  the  sun  than  we  are,  without  being  consumed 
by  his  heat.  Whenever  we  come  nearer  to  any  fire, 
the  heat  rapidly  increases,  being  four  times  as  great 
at  half  the  distance,  and  one  hundred  times  as  great 
at  one  tenth  the  distance.  This  fact  is  expressed  by 
saying,  that  heat  increases  as  the  square  of  the  distance 
decreases.  Our  globe  is  situated  at  such  a  distance 
from  the  sun,  as  exactly  suits  the  animal  and  vegetable 
kingdoms.  Were  it  either  much  nearer  or  more 
remote,  they  could  not  exist,  constituted  as  they  are. 
The  intensity  of  the  solar  light  also  follows  the  same 

that  of  the  fixed  stars  1  Effort  to  form  an  idea  of  great  distances.  How 

long  would  it  take  a  ship,  moving  ten  miles  an  hour,  to  reach  the  sun  1 

223.  Diameter  of  the  sun.    How  many  bodies  like  the  earth  would 

it  take  to  reach  across  the  sun  1    How  long  the  ship  to  sail  over  it  1 

Why  it  appears  no  larger  1    How  would  it  appear  could  we  approach 

nearer  and  nearer  to  it  *    How  is  the  intensity  of  heat  proportioned 

* 


210  ASTRONOMY. 

law.  Consequently,  were  we  much  nearer  the  sun 
than  we  are,  its  blaze  would  be  insufferable  ;  or  were 
we  much  farther  off,  the  light  would  be  too  dim  to  serve 
all  the  purposes  of  vision. 

224.  The  sun  is  one  million  four  hundred  thousand 
(1,400,000)  times  as  large  as  the  earth  ;  but  its  matter 
is  only  about  one  fourth  as  dense  as  that  of  the  earth, 
being  only  a  little  heavier  than  water,  while  the  average 
density  of  the  earth  is  more  than  five  times  that  of 
water.     Still,  on  account  of  the  immense  magnitude 
of  the  sun,  its  quantity  of  matter  is  354,000  times  as 
great  as  that  of  the  earth.     Bodies  would  weigh  about 
twenty-eight  times  as  much  at  the  surface  of  the  sun 
as  they  do  on  the  earth.     Hence,  a  man  weighing  three 
hundred  pounds  would,  if  conveyed  to  the  surface  of 
the  sun,  weigh  8,400  pounds,  or  nearly  three  tons  and 
three  quarters.     A  man's  limb,  weighing  forty  pounds, 
would  require  to  lift  it  a  force  of  1,120  pounds,  which 
would  be  beyond  the  ordinary  power  of  the  muscles. 
At  the  surface  of  the  earth,  a  body  falls  from  rest  by 
the  force  of  gravity,  in  one  second,  16  T'¥  feet ;  but  at 
the  surface  of  the  sun,  a  body  would,  in  the  same  time, 
fall  through  449  feet. 

225.  When  we  look  at  the  sun  through  a  telescope, 
we  commonly  find  on  his  disk  a  greater  or  less  number 
of  dark  places,  called  Solar   Spots.      Sometimes  the 
sun's  disk  is  quite  free  from  spots,  while  at  other  times 
we  may  see  a  dozen  or  more  distinct  clusters,  each 
containing  a  great  number  of  spots,  some  large  and 
some  very  minute.     Occasionally  a  single  spot  is  so 
large  as  to  be  visible  to  the  naked  eye,  especially  when 

to  the  distance  1  Were  the  earth  nearer  the  sun,  what  would  be  the 
consequence  1  How  would  its  light  increase  7 

224.  How  much  larger  is  the  sun  than  the  earth  1    How  much 
greater  is  its  quantity  of  matter  1    How  much  more  would  bodies 
weigh  at  the  sun  than  at  the  earth  1    How  much  would  a  man  of 
three  hundred  pounds  weigh  1    Through  what  space  would  a  body 
fall  in  a  second  1 

225.  Solar  spots — their  number — size  of  the  largest — their  apparent 


THE    SUN.  211 

.;  the  sun  is  near  the  horizon,  and  the  glare  of  his  light 

is  taken  off.     Spots  have  been  seen  more  than  50,000 

miles   in   diameter.      They  move   slowly   across  the 

|  central  regions  of  the  sun.    As  they  have  all  a  common 

motion  from  day  to  day  across  the  sun's  disk ;  as  they 

i  go  off  on  one  limb,  and,  after  a  certain  interval,  some- 

I  times  come  on  again  on  the  opposite  limb,  it  is  inferred 

that  this  apparent  motion  is  imparted  to  them  by  an 

actual   revolution  of  the  sun  on   his   axis,  which   is 

<  accomplished  in  about  twenty-five  days.    This  is  called 

:  the  sun's  diurnal  revolution,  while  his  apparent  move- 

I  ment  about  the  earth  once  a  year  is  called  his  annual 

J  revolution. 

226.  We  have  seen  that  the  apparent  revolution  of 

I  the  heavenly  bodies,  from  east  to  west,  every  twenty- 

i  four  hours,  is  owing  to  a  real  revolution  of  the  earth  on 

I  its  own  axis  in  the  opposite  direction.     This  motion  is 

very  easily  understood,   resembling,   as   it   does,  the 

!  spinning  of  a  top.     We  must,  however,  conceive  of  the 

Jtop  as  turning  without  any  visible  support,  and  not  as 

resting  in  the  usual  manner  on  a  plane.     The  annual 

j -motion  of  the  earth  around  the  sun,  which  gives  rise  to 

!  the  apparent  motion  of  the  sun  around  the  earth  once  a 

year,  is  somewhat  more  difficult  to  understand.    When, 

as  the  string  is  pulled,  the  top  is  thrown  forward  on 

|  the  floor,  we  may  see  it  move  onward  (sometimes  in  a 

:  circle)  at  the  same  time  that  it  spins  on  its  axis.     Let 

a  candle  be  placed  on  a  table,  to  represent  the  sun, 

and  let  these  two  motions  be  imagined  to  be  given 

to  a  top  around  it,  and  we  shall  have  a  case  somewhat 

resembling  the  actual  motions  of  the  earth  around  the 

sun. 

i 

.;  motions — revolution  of  the  sun.     Distinction  between  the  diurnal 
j  and  annual  revolutions  of  the  sun. 

226.  To  what  is  the  apparent  daily  motion  of  the  sun  from  east 
I  to  west  owing  1  How  to  conceive  of  it  1  How  to  conceive  of  the 
I  annual  motion  1 


212  ASTRONOMY. 

227.  When  bodies  are  at  such  a  distance  from  each 
other  as  the  earth  and  sun,  a  spectator  on  either  would 
project  the  other  body  upon  the  face  of  the  sky,  always 
seeing  it  on  the  opposite  side  of  a  great  circle,  one 
hundred  and  eighty  degrees  from  himself.  Let  Fig. 
Fig.  102. 


102  represent  the  relative  positions  of  the  earth  and 
sun,  and  the  firmament  of  stars.  A  spectator  on  the 
earth  at  °|°,  (Aries,)  would  see  the  sun  in  the  heavens  - 
at  =£:,  (Libra  ;)  and  while  the  earth  was  moving  from 
°|°  to  o5,  (Cancer,)  not  being  conscious  of  our  own,-> 
motion,  but  observing  the  sun  to  shift  his  apparent  place 
from  £.  to  VS,  (Capricornus,)  we  should  attribute  the 
change  to  a  real  motion  in  the  sun,  and  infer  that  the 
sun  revolves  about  the  earth  once  a  year,  and  not  the 
earth  about  the  sun.  Although  astronomers  have 
learned  to  correct  this  erroneous  impression,  yet  they 
still,  as  a  matter  of  convenience,  speak  of  the  sun's 
annual  motion. 


227.  How  would  a  spectator  on  the  sun  or  the  earth,  project  the 
other  body  1    Illustrate  by  the  Figure. 


THE   SUN.  213 

228.  In  endeavoring  to  obtain  a  clear  idea  of  the 
revolution  of  the  earth  around  the   sun,   imagine  to 
yourself  a  plane  (a  geometrical  plane,  having  merely 
length  and  breadth,  but  no  thickness,)  passing  through 
the  centers  of  the  sun  and  earth,  and  extended  far  be-< 
yond  the  earth,  until  it  reaches  the  firmament  of  stars. 
This  is  the  plane  of  the  ecliptic  ;  the  circle  in  which 
it  seems  to  cut  the  heavens  is  the  celestial  ecliptic  ;  and 
the  path  described  by  the  earth  in  its  revolution  around 
,the  sun,  is  the  earth's  orbit.     This  is  to  be  conceived 
of  as  near  to  the  sun  compared  with  the  celestial  eclip- 
tic, although  both  are  in  the  same  plane.     Moreover, 
we  project  the  sun  into  the  celestial  ecliptic,  because 
»it  seems  to  travel  along  the  face  of  the  starry  heavens, 
I  i  since  the  sun  and  stars  are  both  so  distant  that  we  ean- 
jnot  distinguish  between  them  in  this  respect,  but  see 
jthem  both  as  if  they  were  situated  in  the  imaginary 
idome  of  the  sky.     If  the  sun  left  a  visible  trace  on 
Ithe  face  of  the  sky,  the  celestial  ecliptic  would   of 
•course  be  distinctly  marked  on  the  celestial  sphere,  as 
jit  is  on  an  artificial  globe  ;  and  were  the  celestial  equa- 

tor delineated  in  a  similar  manner,  we  should  then  see, 
at  a  glance,  the  relative  position  of  these  two  circles  ; 
the  points  where  they  intersect  one  another  constituting 
ithe  equinoxes;  the  points  where  they  are  at  the  greatest 
(distance  asunder,  being  the  solstices  ;  and  the  angle 
!  which  the  two  circles  make  with  each  other,  (23°  28',) 
being  the  obliquity  of  the  ecliptic. 

229.  As  the  earth  traverses  every  part  of  her  orbit 
sin  the  course  of  a  year,  she  will  be  once  at  each  sol- 
stice, and,  once  at  each  equinox.     The  best  way  of 


228.  To  obtain  a  clear  idea  of  the  revolution  of  the  earth  around 
ithe  sun,  what  device  shall  we  employ  1  What  is  the  plane  of  the 
1  ecliptic  1  What  the  celestial  ecliptic  *  What  the  earth's  orbit  1 
Into  what  do  we  project  the  sun  1  If  the  sun  left  a  visible  track. 
:  what  would  it  mark  out  1  If  the  celestial  equator  were  delineated 
;  in  the  same  way,  what  would  it  mark  out  J  Where  would  be  the 
1  equinoxes  —  the  solstices  1  What  is  the  obliquity  1 


214  ASTRONOMY. 

obtaining  a  correct  idea  of  her  two  motions,  is  to  con- 
ceive  of  her  as  standing  still  a  single  day,  at  some 
point  in  her  orbit,  until  she  has  turned  once  on  her 
axis,  then  moving  about  a  degree,  and  halting  again 
until  another  diurnal  revolution  is  completed.  Let  us 
suppose  the  earth  at  the  Autumnal  Equinox,  the  sun,  of 
course,  being  at  the  Vernal  Equinox.  Suppose  the 
earth  to  stand  still  in  its  orbit  for  twenty-four  hours. 
The  revolution  of  the  earth  on  its  axis,  in  this  period, 
from  west  to  east,  will  make  the  sun  appear  to  de- 
scribe a  great  circle  of  the  heavens  from  east  to  west, 
coinciding  with  the  equator.  At  the  end  of  this  time, 
suppose  the  sun  to  move  northward  one  degree  in  its 
orbit,  and  to  remain  there  twenty-four  hours,  in  which 
time  the  revolution  of  the  earth  will  make  the  sun  ap- 
pear to  describe  another  circle  from  east  to  west,  but 
a  little  north  of  the  equator.  Thus,  we  may  conceive 
of  the  sun  as  moving  one  degree  in  the  northern  half 
of  its  orbit,  every  day,  for  about  three  months,  when.1 
he  will  reach  the  point  of  the  ecliptic  farthest  from  the 
equator,  which  point  is  called  the  tropic,  from  a  Greek 
word  signifying  to  turn;  because,  after  the  sun  has 
passed  this  point,  his  motion  in  his  orbit  carries  him 
continually  toward  the  equator,  and  therefore  he  seems 
to  turn  about.  The  same  point  is  also  called  the  sol- 
stice, from  a  Latin  word  signifying  to  stand  still;  since, 
when  the  sun  has  reached  its  greatest  northern  or ' 
southern  limit,  he  seems  for  a  short  time  stationary,  with 
regard  to  his  annual  motion,  appearing  for  several  days 
to  describe,  in  his  daily  motion,  the  same  parallel  of 
latitude. 

230.  When  the  sun  is  at  the  northern  tropic,  which 
happens  about  the  21st  of  June,  his  elevation  above 
the  southern  horizon  at  noon  is  the  greatest  in  the 

229.  How  to  obtain  a  clear  idea  of  the  earth's  two  motions — de- 
scribe the  process — why  is  the  turning  point  called  the  tropic? 
Why  the  solstice  1 


THE   SUN.  215 

year ;  and  when  he  is  at  the  southern  tropic,  about 

the  21st  of  December,  his  elevation  at  noon  is  the  least 

in  the  year. 

231.  The  motion  of  the  earth,  in  its  orbit,  is  nearly 
!  seventy  times  as  great  as  its  greatest  motion  around 
'its  axis.  In  its  revolutions  around  the  sun,  the  earth 
,  moves  no  less  than  1,640,000  miles  a  day,  68,000 
.miles  an  hour,  1,100  miles  a  minute,  and  19  miles 
j  every  second — a  velocity  sixty  times  as  great  as  the 
i  greatest  velocity  of  a  cannon  ball.  Places  on  the 

earth  turn  with  very  different  degrees  of  velocity  in 
;  different  latitudes.  Those  on  the  equator  are  carried 
|  round  at  the  rate  of  about  1000  miles  an  hour.  In 
jour  latitude,  (41°  18',)  the  diurnal  velocity  is  about 
j  750  miles  an  hour.  It  would  seem  at  first  quite  in- 
j  credible  that  we  should  be  whirled  round  at  so  rapid 

a  rate,  and  yet  be  entirely  insensible  of  any  motion ; 

and  much  more  that  we  should  be  going  on  so  swiftly 
j  through  space,  in  our  circuit  around  the  sun,  while 
J  all  things,  when  unaffected  by  local  causes,  appear  to 
•  be  in  such  a  state  of  quiescence.  Yet  we  have  the 
|  most  unquestionable  evidence  of  the  fact ;  nor  is  it 
j  difficult  to  account  for  it,  in  consistency  with  the  gene- 
1 I  ral  state  of  repose  among  bodies  on  the  earth,  when 
4  we  reflect  that  their  relative  motions,  with  respect  to 
j  each  other,  are  not  in  the  least  disturbed  by  any  mo- 
|  tions  which  they  may  have  in  common.  When  we 
j  are  on  board  a  steamboat,  we  move  about  in  the  same 
i  manner  when  the  boat  is  in  rapid  motion,  as  when  it 
|  is  lying  still ;  and  such  would  be  the  case,  if  it  moved 

steadily  a  hundred  times  faster  than  it  does.     Were 

230.  When  does  the  sun  reach  the  northern  tropic  *?    How  is  then 
i  his  altitude  1    When  is  he  at  the  southern  tropic  1    His  altitude 
1  then  * 

231.  How  much  greater  is  the  motion  of  the  earth  in  its  orbit  than 
i  on  its  axis  1    How  many  miles  per  day— per  hour— per  minute — per 
'  second  1    Rates  of  motion  of  places  in  different  latitudes  1    Rate  in 

latitude  41  degrees  and  18  minutes  1  Why  are  we  insenbible  to  this 


216  ASTRONOMY. 

the  earth,  however,  suddenly  to  stop  its  diurnal  motion, 
all  movable  bodies  on  its  surface  would  be  thrown  off 
in  tangents  to  the  surface,  with  velocities  proportional 
to  that  of  their  diurnal  motion ;  and  were  the  earth 
suddenly  to  halt  in  its  orbit,  we  should  be  hurled  for. 
ward  into  space  with  inconceivable  rapidity. 

232.  The  phenomena  of  the  SEASONS,  which  we 
may  now  explain,  depend  on  two  causes  ;  first,  the' 
inclination  of  the  earth's  axis  to  the  plane  of  its  orbit ; 
and,  secondly,  to  the  circumstance  that  the  earth's  axis 
always  remains  parallel  to  itself.  Imagine  a  candle, 
placed  in  the  center  of  a  large  ring  of  wire,  to  repre- 
sent the  sun  in  the  center  of  the  earth's  orbit,  and  an 
apple  with  a  knitting-needle  running  through  it,  in  thai 
direction  of  the  stem.  Run  a  knife  round  the  central, 
part  of  the  apple,  to  mark  the  situation  of  the  equator. 
The  circumference  of  the  ring  represents  the  earth's 
orbit  in  the  plane  of  the  ecliptic.  Place  the  apple  so 
that  the  equator  shall  coincide  with  the  wire ;  then 
the  axis  will  lie  directly  across  the  plane  of  the  eclip- 
tic ;  that  is,  at  right  angles  to  it.  Let  the  apple  be| 
carried  quite  round  the  ring,  constantly  preserving  the] 
axis  parallel  to  itself,  and  the  equator  all  the  while] 
coinciding  with  the  wire  that  represents  the  orbitJ 
Now,  since  the  sun  enlightens  half  the  globe  at  once, 
so  the  candle,  which  here  represents  the  sun,  will 
shine  on  the  half  of  the  apple  that  is  turned  toward! 
it ;  and  the  circle  which  divides  the  enlightened  fromj 
the  unenlightened  side  of  the  apple,  called  the  termi-\ 
nator,  will  pass  through  both  the  poles.  If  the  applej 
be  turned  slowly  round  on  its  axis,  the  terminator  will; 
pass  successively  over  all  places  on  the  earth,  giving! 

great  motion  1  Illustrate  by  a  steamboat.  What  would  be  the 
consequence  were  the  earth  suddenly  to  stop  its  motions  1 

232.  What  are  the  two  causes  of  the  change  of  seasons!  Howl 
illustrated  1  How  will  the  appearances  be  when  the  apple  is  so] 
placed  that  its  equator  coincides  with  the  wire  1  Where  will  it  be| 
sunrise — where  sunset  1 


THE    SUN.  217 

the  appearance  of  sunrise  to  places  at  which  it  arrives, 
and  of  sunset  to  places  from  which  it  departs. 

233.  If,  therefore,  the  earth's  axis  had  been  perpen- 
dicular to  the  plane  of  its  orbit,  in  which  case  the  equator 
would  have  coincided  with  the  ecliptic,  the  diurnal 
motion  of  the  sun  would  always  have  been  in  the  equator, 
and  the  days  and  nights  would  have  been  equal  all  over 
the  globe,  and  there  would  have  been  no  change  of 
seasons.     To  the  inhabitants  of  the  equatorial  regions, 
the  sun  would  always  have  appeared  to  move  in  the 
prime  vertical,  rising  directly  in  the  east,  passing  through 
the  zenith  at  noon,  and  setting  in  the  west.    In  the  polar 
regions,  the  sun  would  always  have  appeared  to  revolve 
in  the  horizon  ;  while,  at  any  place  between  the  equator 
and  the  pole,  the  course  of  the  sun  would  have  been 
oblique  to  the  horizon,  but  always  oblique  in  the  same 
degree.      There  would    have  been  nothing  of  those 
agreeable  vicissitudes  of  the  seasons  which  we  now 
enjoy ;   but  some  regions  of  the  earth  would  have  been 
crowned  with  perpetual  spring  ;  others  would  have 
been  scorched  with  a  burning  sun  continually  over- 
head  ;    while   extensive   regions  toward    either   pole, 
would   have  been  consigned  to  everlasting  frost  and 
barrenness. 

234.  In  order  to  simplify  the  subject,  we  have  just 
supposed  the  earth's  axis  to  be  perpendicular  to  the 
plane  of  its  orbit,  making  the  equator  to  coincide  with 
the  ecliptic  ;  but  now,  (using  the  same  apparatus  as 
before,)  turn  the  apple  out  of  a  perpendicular  position 
a  little,  (231  degrees,)  then  the  equator  will  be  turned 
just  the  same  number  of  degrees  out  of  a  coincidence 
with  the  ecliptic.    Let  the  apple  be  carried  around  the 


233.  Comparative  lengths  of  the  days  and  nights  *    Appearances 
to  the  inhabitants  of  the  equatorial  regions  1    Of  the  polar  regional 
Would  there  have  been  any  change  of  seasons  1 

234.  Repeat  the  process  with  the  axis  inclined.    How  far  would 
the  equator  be  turned  out  of  a  coincidence  with  the  ecliptic  1    How 

19 


218 


ASTRONOMY. 


ring,  always  holding  it  inclined  at  the  same  angle  to 
the  plane  of  the  ring,  and  always  parallel  to  itself,  as 


in  figure  103.  We  shall  find  that  there  are  two  points, 
A  and  C,  in  the  circuit,  where  the  light  of  the  sun 
(which  always  enlightens  half  the  globe  at  once) 
reaches  both  poles.  These  are  the  points  where  the 
celestial  equator  and  ecliptic  cut  one  another,  or  the 
equinoxes.  When  the  earth  is  at  either  of  these  points, 
the  sun  shines  on  both  poles  alike ;  and  if  we  conceive 
of  the  earth,  while  in  this  situation,  as  turning  once 
round  on  its  axis,  the  apparent  diurnal  motion  of  the 
sun  would  be  the  same  as  it  would  be,  were  the  earth's 
axis  perpendicular  to  the  plane  of  the  equator.  For 
that  day,  the  earth  would  appear  to  revolve  in  the  .] 
equator,  and  the  days  and  nights  would  be  equal  all 
over  the  globe. 

235.  If  the  apple  were  carried  round  in  the  manner 
supposed,  then,  at  the  distance  of  ninety  degrees  from  ] 
the  equinoxes,  at  B  and  D,  the  same  pole  would  bel 
turned  toward  the  sun  on  one  side,  just  as  much  as  it 


does  the  sun  then  shine  with  respect  to  the  poles  1    What  will  then 
fee  the  appearances  in  the  diurnal  motion  1 


THE   SUN.  219 

was  turned  from  him  on  the  other.  In  the  former  case, 
the  sun's  light  would  reach  beyond  the  pole  23^  degrees, 
and  in  the  other  case,  it  would  fall  short  of  it  the  same 
number  of  degrees.  Now  imagine,  again,  the  earth 
turning  in  the  daily  revolution,  and  it  will  be  readily 
seen  how  places  within  23^  degrees  of  the  enlightened 
pole,  will  have  continual  day,  while  places  within  the 
same  distance  of  the  unenlightened  pole,  will  have 
j  continual  night.  By  an  attentive  inspection  of  figure 
1 103,  all  these  things  will  be  clearly  understood.  The 
earth's  axis  is  represented  as  prolonged,  both  to  show 
I  its  position,  and  to  indicate  that  it  always  remains 
parallel  to  itself.  On  March  21st  and  September  22d, 
when  the  earth  is  at  the  equinoxes,  the  sun  shines  on 
both  poles  alike  ;  while  on  June  21st  and  December 
24th,  when  the  earth  is  at  the  solstices,  the  sun  shines 
23^  degrees  beyond  one  pole,  and  falls  the  same  distance 
short  of  the  other. 

236.  Two  causes  contribute  to  increase  the  heat  of 
summer  and  the  cold  of  winter, — the  changes  in  the 
sun's  meridian  altitudes  >  and  in  the  lengths  of  the  days. 
The  higher  the  sun  ascends  above  the  horizon,  the  more 
directly  his  rays  fall  upon  the  earth ;  and  their  heating 
power  is  rapidly  increased  as  they  approach  a  perpen- 
dicular direction.  The  increased  length  of  the  day  in 
jsummer,  affects  greatly  the  temperature  of  places 
toward  the  poles,  because  the  inequality  between  the 
lengths  of  the  day  and  night  is  greater  in  proportion 
as  we  recede  from  the  equator.  By  the  operation  of 
{this  cause,  the  heat  accumulates  so  much  in  summer, 
that  the  temperature  rises  to  a  higher  degree  in  mid- 
summer, at  places  far  removed  from  the  equator,  than 
within  the  torrid  zone. 

235  At  the  distance  of  90  degrees  from  the  equinoxes,  how  would 
the  sun  shine  with  reppect  to  tfie  poles'? 

236  What  two  causes  contribute  to  increase  the  heat  of  summer 
and  the  ^old  of  winter  1  Effect  of  the  sun's  altitude — of  the  increased 
length  of  the  day  1 


820  ASTRONOMY. 


: 


237.  But  the  temperature  of  a  place  is  influence' 
very  much  by  several  other  causes,  as  well  as  by  th 
force  and  duration  of  the  sun's  heat.     First,  the  eleva- 
tion of  a  country  above  the  level  of  the  sea,  has  a 
great  influence  upon  its  climate.     Elevated  districts  of  ; 
country,  even  in  the  torrid  zone,  often  enjoy  the  most! 
agreeable  climate  in  the  world.    The  cold  of  the  upper  j 
regions  of  the  atmosphere  modifies  and  tempers  the  j 
solar  heat,  so  as  to  give  a  most  delightful  softness,  while  i. 
the  uniformity  of  temperature  excludes  those  sudden 
and  excessive  changes  which  are  often  experienced  in 
less  favored  climes.     In  ascending  high  mountains, 
situated  within  the  torrid  zone,  the  traveller  passes,  hv 
a  short  time,  through  every  variety  of  climate,  from  < 
the  most  oppressive  and  sultry  heat,  to  the  soft  and '' 
balmy  air  01  spring,  which  again  is  succeeded  by  the 
cooler  breezes  of  autumn,  and  then  by  the  severest 
frosts  of  winter.    A  corresponding  difference  is  seen  in  > 
the  products  of  the  vegetable  kingdom.     While  winter 
reigns  on  the  summit  of  the  mountain,  its  central  regions  ; 
may  be  encircled  with  the  verdure  of  spring,  and  itsj^ 
base  with  the  flowers  and  fruits  of  summer.    Secondly, 
the  vicinity  of  the  ocean  has  also  a  great  effect  to;1 
equalize  the  temperature  of  a  place.     As  the  ocean 
changes  its  temperature  during  the  year  much  less  than  a 
the  land,  it  becomes  a  source  of  warmth  to  neighboring! 
countries  in  winter,  and  a  fountain  of  cool   breezes  inA 
summer.     Thirdly,  the  relative  moisture  or  dryness  off1' 
the  atmosphere  of  a  place  is  of  great  importance,  in' 
regard  to  its  effects  on  the  human  system.     A  dry  air|| 
of  ninety  degrees,  is  not  so  insupportable  as  a  moisli 
air  of  eighty  degrees.     As  a  general  principle,  a  hot  ' 
and  moist  air  is  unhealthy,  although  a  hot  air,  when 
dry,  may  be  very  salubrious. 

237.  Effect  of  elevation— of  the  vicinity  of  the  ocean— relative 
moisture  and  dryness. 


CHAPTER  V. 
THE  MOON. 

DISTANCE     AND     DIAMETER APPEARANCES     TO     THE     TELESCOPE 

MOUNTAINS    AND    VALLEYS REVOLUTION ECLIPSES TIDES. 

238.  THE  Moon  is  a  constant  attendant  or  satellite 
of  the  earth,  revolving  around  it   at  the  distance  of 
about  240,000  miles.     Her  diameter  exceeds  2,000 
miles,  (2160.)     Her  angular  breadth  is  about  half  a 
degree, — a  measure  which  ought  to  be  remembered, 
as  it  is  common  to  estimate  fire-balls,  and  other  sights 
in  the  sky,  by  comparing  them  with  the  size  of  the 
moon.     The  sun's  angular  diameter  is  a  little  greater. 

239.  When  we  view  the  moon  through  a  good  tel- 
escope, the  inequalities  of  her  surface  appear  much 
more  conspicuous  than  to  the  naked  eye ;  and  by  stu- 
dying them  attentively,  we  see  undoubted  proofs  that 
the  face  of  the  moon  is  very  rough  and  broken,  exhib- 

\  iting  high  mountains  and  deep  valleys,  and  long  moun- 
tainous ridges.  The  line  which  separates  the  enlight- 
ened from  the  dark  part  of  the  moon,  is  called  the 
\  Terminator.  This  line  appears  exceedingly  jagged, 
\  indicating  that  it  passes  over  a  very  broken  surface 
of  mountains  and  valleys.  Mountains  are  also  indi- 
cated by  the  bright  pointe  and  crooked  lines,  which 
lie  beyond  the  terminator,  within  the  unilluminated 
part  of  the  moon ;  for  these  can  be  nothing  else  than 
elevations  above  the  general  level,  which  are  enlight- 
ened by  the  sun  sooner  than  the  surrounding  countries, 
as  high  mountains  on  the  earth  are  tipped  with  the 
morning  light  sooner  than  the  countries  at  their  bases. 
Moreover,  when  these  pass  the  terminator,  and  come 

238.  Of  what  is  the  moon  a  satellite  1  Distance  from  the  earth — 
diameter — angular  breadth.  Why  is  it  important  to  remember  this  *? 

239  How  does  the  moon  appear  to  the  telescope  1  What  is  the 
Terminator  1  How  does  it  appear  1  What  does  its  unevenness  in- 
dicate 1  What  signs  of  mountains  are  there  in  the  dark  part  of  the 
19* 


222 


ASTRONOMY. 


within  the  enlightened  part  of  the  disk,  they  are  fur- 
ther recognised  as  mountains,  because  they  cast  shad- 
ows opposite  the  sun,  which  vary  in  length  as  the  sun 
strikes  them  more  or  less  on  a  level. 


Fig.  104. 


240.  Spots,  also,  on  the  lunar  disk,  are  known  to  be 
Valleys,  because  they  exhibit  the  same  appearance  as 
is  seen  when  the  sun  shines  into  a  tea  cup,  when  it 
strikes  it  very  obliquely.  The  inside  of  the  cup,  oppo* 
site  to  the  sun,  is  illuminated  in  the  form  of  a  crescent, 


moon'?    When  the  terminator  passes  beyond  these^  what  signs  01 
being  mountains  do  they  give  1 
240.  Valleys,  how  known.    Illustrate  by  the  mode  in  which  lighl 


THE  MOON.  223 

(as  every  one  may  see,  who  will  take  the  trouble  to 
try  the  experiment,)  while  the  inside,  next  the  sun, 
casts  a  deep  shadow.     Also,  if  the  cup  stands  on  a 
table,  the  side  farthest  from  the  sun  casts  a  shadow 
on  the  table  outside  of  the  cup.     Similar  appearances, 
presented  by  certain  spots  in  the  moon,  indicate  very 
clearly  that  they  are  valleys.     Many  of  them  are  reg- 
ular circles,  and  not  unfrequently  we  may  see  a  chain 
of  mountains,  surrounding  a  level  plain  of  great  ex- 
tent, from  the  center  of  which  rises  a  sharp  mountain, 
casting  its  shadow  on    the   plain   within   the   circle. 
Figure  104  is  an  accurate  representation  of  the  tele- 
scopic appearance  of  the  moon   when  five  days  old. 
!  It  will  be  seen  that  the  terminator  is  very  uneven,  and 
that  white  points  and   lines  within  the  unenlightened 
part   of  the  disk,  indicate  the  tops  of  mountains  and 
j  mountain  ridges.     Near  the  bottom  of  the  terminator, 
i  a  little  to  the  left,  we  see  a  small  circular  spot,  sur- 
I  rounded  by  a  high  chain  of  mountains,  (as  is  indicated 
j  by  the  shadows  they  cast,)  and  in  the  center  of  the 
I  valley  the  long  shadow  of  a  single  mountain  thrown 
upon  the  plain.     Just  above  this  valley,  we  see  a  ridge 
of  mountains,  casting  uneven  shadows  opposite  to  the 
sun,  some  sharp,  like  the  shadows  of  mountain  peaks. 
These  appearances  are,  indeed,  rather  minute  ;  but  we 
j  must  recollect  that  they  are  represented  on  a  very 
j  small  scale.     The  most  favorable  time  for  viewing  the 
I  mountains  and  valleys  of  the  moon  with  a  telescope, 
I  is  when  she  is  about  seven  days  old. 

241.  The  full  moon  does  not  exhibit  the  broken  as- 
*  pect  so  well  as  the  new  moon ;  but  we  see  dark  and 
|  light  regions  intermingled.  The  dusky  places  in  the 
|  moon  were  formerly  supposed  to  consist  of  water,  and 

|  shines  into  a  cup.    What  shape  have  many  of  the  valleys  1    What 
<>}  do  we  sometimes  see  surrounding  the  valley  1    What  rising  in  the 
£  center  of  it  1    Point  out  mountains  and  valleys  on  the  diagram. 
241.  What  is  said  of  the  telescopic  view  of  the  full  moon  1  What 


224  ASTRONOMY. 

the  brighter  places,  of  land ;  astronomers,  however, 
are  now  of  the  opinion,  that  there  is  no  water  in  the 
.moon,  but  that  the  dusky  parts  are  extensive  plains, 
while  the  brightest  streaks  are  mountain  ridges.  Each 
separate  place  has  a  distinct  name.  Thus,  a  remark- 
able spot  near  the  top  of  the  moon,  is  called  Tycho ; 
another,  Kepler ;  and  another,  Copernicus ;  after  cel- 
ebrated astronomers  of  these  names.  The  large 
dusky  parts  are  called  seas,  as  the  Sea  of  Humors, 
the  Sea  of  Clouds,  and  the  Sea  of  Storms.  Some  of 
the  mountains  are  estimated  as  high  as  five  miles,  and 
some  of  the  valleys  four  miles  deep. 

242.  The  moon  revolves  about  the  earth  from  west 
to   east,  once   a  month,   and    accompanies   the  earth  ; 
around  the  sun  once  a  year.     The  interval  in  which 
she   goes  through  the  entire  circuit  of  the  heavens, 
from  any  star  round  to  the  same  star  again,  is  called  a 
sidereal  month,  and  consists  of  about  27£  days  ;  but  the 
time  which  intervenes  between  one  new  moon    and 
another,  is  called  a  synodical  month,  and  is  composed 
of  29^  days.     A  new  moon  occurs  when  the  sun  and, 
moon  meet  in  the  same  part  of  the  heavens ;  for  al- 
though the  sun  is  400  times  as  distant  from  us  as  the 
moon,  yet  as  we  project  them  both  upon  the  face  of  the 
sky,  the  moon  seems  to  be  pursuing  her  path  among 
the  stars  as  well  as  the  sun.     Now  the  sun,  as  well  as 
the   moon,  is  travelling  eastward,  but  with  a  slower 
pace  ;  the  sun  moves  only  about  a  degree  a  day,  while  ' 
the   moon  moves  more  than  thirteen  degrees  a  day.  ' 
While  the  moon,  after  being  with  the  sun,  has  been  •' 
going  round  the  earth  in  27£  days,  the  sun,  mean-': 

were  the  dark  places  in  the  moon  formerly  supposed  to  be  *?  What 
do  astronomers  now  consider  them  1  How  are  places  on  the  moon 
named  1  Repeat  some  of  the  names.  What  is  the  height  of  some 
of  the  mountains,  and  depth  of  the  valleys'? 

242  Revolutions  of  the  moon.  What  is  a  sidereal  month  1  How 
long  is  it  1  What  is  a  synodical  month  1  When  does  a  new  moon 
occurl  Why  is  the  synodical  longer  than  the  sidereal  month  1 


THE  MOON.  225 

\vhile,  has  been  going  eastward  about  27  degrees ;  so 
that,  when  the  moon  returns  to  the  part  of  the  heavens 
where  she  left  the  sun,  she  does  not  find  him  there,  but 
takes  more  than  two  days  to  catch  up  with  him. 

243.  The  moon,  however,  does  not  pursue  precisely 
I  the  same  track  with  the  sun  in  his  apparent  annual 

motion,  though  she  deviates  but  little  from  his  path. 
The  inclination  of  her  orbit  to  the  ecliptic  is  only 
about  five  degrees,  and,  of  course,  the  moon  is  never 
seen  farther  from  the  ecliptic  than  that  distance, 
,and  she  is  commonly  much  nearer  to  it  than  that. 
:The  two  points  where  the  moon's  orbit  crosses  the 
!  ecliptic,  are  called  her  nodes.  They  are  the  intersec- 
|  tions  of  the  solar  and  lunar  orbits,  as  the  equinoxes  are 
ithe  intersections  of  the  equator  and  ecliptic,  and,  like 
I  the  latter,  are  180  degrees  apart. 

244.  The  changes  of  the  moon,  Fig.  105. 
j  commonly  called  her  phases,  arise 

;  from  different  portions  of  her  en- 

j  lightened  side  being  turned  toward 

jthe  earth  at  different  times.    When 

j  the  moon  comes  between  the  earth 

1  and  the  sun,  her  dark  side  is  turned 

\  toward  us,  and  we  lose  sight  of  her     ^ 

I  for  a  short  period,  at  A,  (Fig.  105,) 

j  when  she  is  said  to  be  in  conjunc- 
tion.    As  soon  as  she  gets  a  little  Cc 
past   conjunction,  at   B,  we   first 
observe  her  in  the   evening  sky,     Q  ^ 

'  in  the    form   of  a    crescent, — the  Jk 

j  well  known  appearance  of  the  new 

j  moon.  When  at  C,  half  her  enlightened  disk  is  turned  to- 

I  ward  us,  and  she  is  in  quadrature,  or  in  her  first  quarter. 


243.  How  many  degrees  is  the  moon's  orbit  inclined  to  the  eclip- 
tic 1    Define  the  nodes.     How  far  apart  1 

!244.  Whence  arise  the  phases  of  the  moon  7    When  is  the  moon 
said  to  be  in  conjunction  1    When  in  quadrature  1    When  in  oppo- 


226  ASTRONOMY. 

At  D,  three-fourths  of  the  disk  is  illuminated,  and  at  E, 
when  the  earth  lies  between  the  sun  and  the  moon,  her 
whole  disk  is  enlightened,  and  she  is  in  opposition,  thi 
time  of  full  moon.  In  proceeding  from  opposition  t 
conjunction,  or  from  full  to  new  moon,  the  illuminated 
portion  diminishes  in  the  same  way  as  it  increased  front 
conjunction  to  opposition,  being  in  the  last  quarter,  at 
G.  Within  the  first  and  last  quarters,  the  terminator 
is  turned  from  the  sun,  and  the  moon  is  said  to  be* 
horned;  but  within  the  second  and  third  quarters,  the' 
terminator  presents  its  concave  side  toward  the  sun,' 
and  the  moon  is  said  to  be  gibbous. 

245.  The  moon  turns  on  her  axis  in  the  same  time* 
in  which  she  revolves  about  the  earth.  This  is  known: 
by  the  moon's  always  keeping  nearly  the  same  facei 
toward  us,  as  is  indicated  by  the  telescope,  which 
could  not  be  the  case,  unless  her  revolution  on  her 
axis  kept  pace  with  her  motion  in  her  orbit.  Take  an 
apple  to  represent  the  moon  :  thrust  a  knitting-needle 
through  it  in  the  direction  of  the  stern,  to  represent  the 
axis,  in  which  case  the  two  eyes  of  the  apple  will  nat- 
urally represent  the  poles.  Through  the  poles,  cut  a 
line  around  the  apple,  dividing  it  into  two  hemispheres, 
and  mark  them  so  as  to  be  readily  distinguished  from 
each  other.  Now  place  a  ball  on  the  table  to  repre- 
sent the  earth,  and  holding  the  apple  by  the  knitting, 
needle,  carry  it  round  the  ball,  and  it  will  be  seen  that, 
unless  the  apple  is  made  to  turn  about  on  its  axis,  as 
it  is  carried  around  the  ball,  it  will  present  different 
sides  toward  the  ball ;  and  that,  in  order  to  make  it? 
always  present  the  same  side,  it  will  be  necessary  to 
make  it  revolve  exactly  once  on  .its  axis,  while  it  is 


sitionl    What  figure  has  the  moon  in  the  first  and  last  quarters! 
What  in  the  second  and  third  1 

245.  In  what  time  does  the  moon  turn  on  her  axis  1  How  is 
this  known  1  How  illustrated  by  an  apple  with  a  knitting-needle  1 
By  walking  round  a  tree  1 


THE  MOON.  227 

gointf  round  the  circle, — the  revolution  on  its  axis 
keeping  exact  pace  with  the  motion  in  its  orbit.  The 
same  thing  will  be  observed,  if  we  walk  around  a  tree, 
always  keeping  the  face  toward  the  tree.  It  will  be 
;  necessary  to  turn  round  on  the  heel  at  the  same  rate 
as  we  go  forward  round  the  tree. 

246.  An  Eclipse  of  the  Moon  happens  when  the 
moon,  in  its  revolution  around  the  earth,  falls  into  the 
earth's  shadow.  An  Eclipse  of  the  Sun  happens  when 
the  moon,  coming  between  the  earth  and  the  sun,  cov- 
ers either  a  part  or  the  whole  of  the  solar  disk.  As 
the  direction  of  the  earth's  shadow  is,  of  course,  op- 
posite  to  the  sun,  the  moon  can  fall  into  it  only  when 
in  opposition,  or  at  the  time  of  full  moon ;  and  as  the 
moon  can  come  between  us  and  the  sun  only  when  in 
conjunction,  or  at  the  time  of  new  moon,  it  is  only 
then  that  a  solar  eclipse  can  take  place.  If  the  moon's 
orbit  lay  in  the  plane  of  the  ecliptic,  we  should  have 
a  solar  eclipse  at  every  new  moon,  and  a  lunar  eclipse 
at  every  full  moon ;  but  as  the  moon's  orbit  is  inclined 
to  the  plane  of  the  ecliptic  about  five  degrees,  the 
moon  may  pass  by  the  sun  on  one  side,  and  the  earth's 
shadow  on  the  other  side,  without  touching  either.  It 
is  only  when,  at  new  moon,  the  sun  happens  to  be  at 
j or  near  the  point  where  the  lunar  orbit  cuts  the  plane 
j  of  the  ecliptic,  or  at  one  of  the  nodes,  that  the  moon's 
disk  overlaps  the  sun's,  and  produces  a  solar  eclipse, 
j  Also,  when  the  sun  is  at  or  near  one  of  the  moon's 
j nodes,  the  earth's  shadow  is  thrown  across  the  other 
[node,  on  the  opposite  side  of  the  heavens,  and  then, 
las  the  moon  passes  through  this  node,  at  the  time  of 
(opposition,  she  falls  within  the  shadow,  and  produces 
ja  lunar  eclipse. 

|  246.  When  does  an  eclipse  of  the  moon  happen  *?  When  an 
I  eclipse  of  the  sun!  At  what  age  of  the  moon  does  it  eclipse  the 
)  sun — and  at  what  age  does  it  suffer  eclipse  1  Why  do  not  eclipses 
i  occur  at  every  revolution  1  At  or  near  what  point  must  the  sun  be, 
I  in  order  that  an  eclipse  may  take  place  1 


228  ASTRONOMY. 

247.  Figure  106  represents  both  kinds  of  eclipses. 
The  shadow  of  the  moon,  when  in  conjunction,  is 
represented  as  just  long  enough  to  reach  the  earth,  as 

Fig.  106. 


is  the  case  when  the  moon  is  at  or  about  her  average 
distance  from  the  earth.  In  this  case,  a  spectator  on 
the  earth,  situated  at  the  place  where  the  point  of  the 
shadow  touches  the  earth,  would  see  the  sun  totally 
eclipsed  for  an  instant,  while  the  countries  around,  for 
a  considerable  distance,  would  see  only  a  partial1 
eclipse,  the  moon  hiding  only  a  part  of  the  sun,  which 
sheds  on  such  places  a  partial  light,  called  the  penum-  * 
bra,  as  is  indicated  in  the  figure  by  the  dark  shading 
on  each  side  of  the  moon's  shadow.  A  similar  pe- 
numbra is  represented  on  each  side  of  the  earth's 
shadow,  because,  when  the  moon  is  approaching  the 
shadow,  a  part  of  the  light  of  the  sun  begins  to  be  in- 
tercepted from  her  when  she  reaches  this  limit,  and 

247.  Describe  Fig.  106.  At  what  point  of  the  earth  would  the 
eclipse  of  the  sun  be  total  1  Where  partial  1  What  is  this  partial 
light  called  1  What,  is  said  of  the  moon's  penumbra  1  What  oc- 
casions an  annular  eclipse  1 


THE   MOON.  229 

she  receives  less  and  less  of  light  from  the  sun,  until, 
when  she  enters  the  shadow,  his  disk  is  entirely  hidden. 
When  the  moon  is  farther  from  the  earth  than  her 
average  distance,  her  disk  is  not  large  enough  to  cover 
the  sun's,  but  a  ring  of  the  sun  appears  all  around  the 
moon,  constituting  an  annular  eclipse. 

248.  Eclipses  of  the  sun  are  more  frequent  than 
those  of  the  moon.     Yet,  lunar  eclipses,  being  visible 
to  every  part,  of  the  hemisphere  of  the  earth  in  which 
the  moon  is  above  the  horizon,  while  those  of  the  sun 
are  visible  only  to  a  small  portion  of  the  hemisphere 
on  which  the  moon's  shadow  falls,  it  happens  that,  for 
any  particular  place  on  the  earth,  there  are  seen  more 
eclipses  of  the  moon  than  of  the  sun.    In  any  year,  the 
number  of  eclipses  of  both  luminaries  cannot  be  less 
than   two,  nor  more  than   seven.      The   most  usual 
number  is  four,  and  it  is  very  rare  to  have  more  than 
six.     A  total  eclipse  of  the  moon  frequently  happens 
at  the  next  full  moon  after  an  eclipse  of  the  sun.    For, 
since,  in  a  solar  eclipse,  the  sun  is  at  or  near  one  of 
the  moon's  nodes, — that  is,  is  projected  to  the  place  in 
the  sky  where  the  moon  crosses  the  ecliptic, — the  earth's 
shadow,  which  is,  of  course,  directly  opposite  to  the 
sun,  must  be  at  or  near  the  other  node,  and  may  not 
have  passed  too  far  from  the  node  before  the  moon 
comes  round  to  the  opposition  and  overtakes  it. 

249.  A  total  eclipse  of  the  sun  is  one  of  the  most 
sublime  and  impressive  phenomena  of  Nature.    Among 
barbarous  tribes,  it  is  always  looked  on  with  fear  and 
astonishment,  and  as  strongly  indicative  of  the  wrath 
of  the  gods.    When  Columbus  first  discovered  America, 

248.  Which  are  most  frequent,  the  eclipses  of  the  sun  or  the  moon  1 
Of  which  are  the  greatest  number  visible  1    What  number  of  both 
can  happen  in  a  single  year  1    What  is  the  most  usual  number  1 
Why  does  an  eclipse  of  the  moon  happen  at  the  next  full  moon 
after  an  eclipse  of  the  sun  1 

249.  What  is  said  of  an  eclipse  of  the  sun  1    What  ia  t9ld  of 
Columbus  1    Why  is  a  total  eclipse  of  the  sun  regarded  with  so 

20 


230  ASTRONOMY. 

and  was  in  danger  of  hostility  from  the  natives,  he  v 
awed  them  into  submission  by  telling  them  that  the  sun 
would  be  darkened  on  a  certain  day,  in  token  of  the;| 
anger  of  the  gods  at  them  for  their  treatment  of  him. : 
Among  cultivated  nations,  also,  a  total  eclipse  of  the  f 
sun  is  regarded  with  great  interest,  as  verifying  with 
astonishing  exactness  the  predictions  of  astronomers, 
and  evincing  the  great  knowledge  they  have  acquired  of 
the  motions  of  the  heavenly  bodies,  and  of  the  laws  by 
which  they  are  governed.     From  1831  to  1838,  was  aj 
period  distinguished  for  great  eclipses  of  the  sun,  ia| 
which  time  there  were  no  less  than  five,  of  the  mos 
remarkable  character.     The  next  total  eclipse  of  the 
sun,  visible  in  the  United  States,  will  occur  on  the  7th  of 
August,  1869. 

250.  Since  Tides  are  occasioned  by  the  influence 
of  the  sun  and  moon,  a  few  remarks  upon  them  will  I 
conclude  the  present  chapter.     By  the  tides  are  meant 
the  alternate  rising  and  falling  of  the  waters  of  the 
ocean.    Its  greatest  and  least  elevations  are  called  high 
and  low  water  ;  its  rising  and  falling  are  called  flood  and 
ebb  ;  and  the  extraordinary  high  and  low  tides  that 
occur  twice  every  month,  are  called  spring  and  neap-  f 
tides.     It  is  high  water,  or  low  water,  on  opposite  sides^ 
of  the  globe  at  the  same  time.    If,  for  example,  we  have-1 
high  water  at  noon,  it  is  also  high  water  to  those  who 
live  on  the  meridian  below  us,  where  it  is  midnight.^ 
In  like  manner,  low  water  occurs  at  the  same  time  on| 
the  upper  and  lower  meridian.    The  average  height  of  1 
the  tides,  for  the  whole  globe,  is  about  two  and  a  ban 
feet ;  but  their  actual  height  at  different  places  is  very", 
various,    sometimes   being   scarcely   perceptible,   and 


much  interest  among  cultivated  nations  1    What  period  was  distin-^ 
guished  for  great  eclipses  of  the  sun  1    When  will  the  next  total 
eclipse  of  the  sun  occur  1 

250.  What  are  the  tides  1    What  is  meant  by  high  and  low  water 
— flood  and  ebb — spring  and  neap  1    Where  is  it  high  water  and 


THE   MOON.  231 

ilsometimes  rising  to  sixty  or  seventy  feet.  In  the  Bay 
rof  Fundy,  where  the  tide  rises  70  feet,  it  comes  in  a 
imighty  wave,  seen  thirty  miles  off,  and  roaring  with  a 
;loud  noise. 

251.  Tides  are  caused  by  the  unequal  attraction  of 
ithe  sun  and  moon  upon  different  parts  of  the  earth.  We 
jshall  attend  hereafter  more  particularly  to  the  subject 
jof  universal  gravitation,  by  which  all  bodies,  or  masses 
jof  matter,  attract  all  other  bodies,  each  according  to  its 
jweight,  when  they  act  on  a  body  at  the  same  distance ; 
but  when  at  different  distances,  the  force  increases 
irapidly  as  the  distance  is  diminished,  so  that  the  force 
of  attraction  is  four  times  as  great  for  half  the  distance, 
jone  hundred  times  as  great  for  one  tenth  the  distance, 
land,  universally,  the  force  increases  in  proportion  as 
ithe  square  of  the  distance  diminishes.  Such  a  force  as 
I  this  is  exerted  by  the  moon  and  by  the  sun  upon  the 
earth,  and  causes  the  tides.  As  the  sun  has  vastly 
more  matter  than  the  moon,  it  would  raise  a  higher  tide 
I  than  the  moon,  were  it  not  so  much  farther  off.  This 
latter  circumstance  gives  the  advantage  to  the  moon, 
which  has  three  times  as  much  influence  as  the  sun  in 
raising  the  tides.  If  these  bodies,  one  or  both  of  them, 
acted  equally  on  all  parts  of  the  earth,  they  would  draw 
i  all  parts  toward  them  alike,  but  would  not  at  all  disturb 
the  mutual  relation  of  the  parts  to  each  other,  and,  of 
course,  would  raise  no  tide.  But  the  sun  or  moon 
attracts  the  water  on  the  side  nearest  to  it  more  than 
the  water  more  remote,  and  thus  raises  them  above  the 
general  level,  forming  the  tide  wave,  which  accompanies 
I  the  moon  in  her  daily  revolution  around  the  earth.  It 
is  not  difficult  to  see  how  the  tide  is  thus  raised  on  the 

where  low  water  at  the  same  time  1  Average  height  of  the  tides 
for  the  whole  globe.  What  is  said  of  their  actual  height  at  different 
places  1  How  high  does  the  tide  rise  in  the  Bay  of  Fundy  * 

251.  By  what  are  tides  caused  1  What  force  is  exerted  by  the  sun 
and  moon  upon  the  earth  1  Why  does  not  the  sun  raise  a  greater 
tide  than  the  moon  1  How  does  the  sun's  greater  distance  give  the 


232  ASTRONOMY. 

side  of  the  meridian  nearest  to  the  moon  ;  but  it  may 
not  be  so  clear  why  a  tide  should  at  the  same  time  be 
raised  on  the  opposite  meridian.    The  reason  of  this  is, 
that  the  waters  farthest  from  the  moon,  being  attracted 
less  than  those  that  are  nearer,  and  less  than  the  solid 
earth,  are  left  behind,  or  appear  to  rise  in  a  direction 
opposite  to  the  center  of  the  earth.     Hence,  we  have  | 
two   tides   every  twenty-four   hours, — one  when   the  ] 
moon  passes  the  upper  meridian,  and  one  when  she 
passes   the    lower.      Each,    however,    is   about    fifty 
minutes   later  to-day  than  yesterday,   for   the    moon  i 
comes  to  the  meridian  so  much  later  on  each  following  • 
day. 

252.  Were  it  not  for  the  impediments  which  prevent 
the  force  from  producing  its  full  effects,  we  might  expect 
to  see  the  great  tide  wave  always  directly  beneath  the 
moon,  attending  it  regularly  around  the  globe.     But  the 
inertia  of  the  waters  prevents  their  instantly  obeying 
the  moon's  attraction,  and  the  friction  of  the  waters  on 
the  bottom  of  the  ocean  still  further  regards  its  progress. 
It  is  not,  therefore,  until  several  hours  after  the  moon 
has  passed  the  meridian  of  a  place,  that  it  is  high  tide 
at  that  place. 

253.  The  sun  has  an  action  similar  to  that  of  the 
moon,  but  only  one  third  as  great.     It  is  not  that  the  » 
moon  actually  exerts  a  greater  force  of  attraction  upon  j 
the  earth  than  the  sun  does,  that  her  influence  in  raising  * 
the  tides  exceeds  that  of  the  sun.     She,  in  fact,  exerts (\ 
much  less  force.     But,  being  so  near,  the  difference  off 
her  attraction  on  different  parts  of  the  earth  is  greater 
than  the  difference  of  the  sun's  attraction  ;   for  the 
sun   is   so   far   off,   that    the   diameter   of  the   earth 

advantage  to  the  moon'?  Why  is  it  high  tide  on  opposite  sides  of-^ 
the  earth  at  the  same  time  1  How  much  later  is  the  high  tide  of  I 
to-day  than  that  of  yesterday  1 

252.  Why  is  it  not  high  tide  when  the  moon  is  on  the  meridian  1 

253.  How  much  less  is  the  action  of  the  sun  in  raising  the  tides 
than  that  of  the  moon  1  Why  has  the  moon  so  much  greater  power  1 


THE  MOON.  233 

bears  but  a  small  proportion  to  the  distance,  and  there- 
fore the  force  exerted  by  tfre  sun  is  more  nearly  equal 
on  all  parts  of  the  earth,  and  we  must  bear  in  mind 
that  the  tides  are  owing,  not  to  the  amount  of  the  force 
of  attraction,  but  to  the  difference  of  the  forces  exerted 
on  different  parts  of  the  earth. 

254.  As  the  sun  and  moon  both  contribute  to  raise 
the  tides,  and  as  they  sometimes  act  together  and 
sometimes  in  opposition  to  each  other,  so  correspond- 
ing variations  occur  in  the  height  of  the  tides.  The 
spring  tides,  or  those  which  rise  to  an  unusual  height 


Fig.  107. 


twice  a  month,  are  produced  by  the  sun  and  moon's 
acting  together ;  and  the  neap  tides,  or  those  which 
are  unusually  low  twice  a  month,  are  produced  by  the 
sun  and  moon's  acting  in  opposition  to  each  other. 
The  spring  tides  occur  when  the  sun  and  moon  act  in 
the  same  line,  as  is  the  case  both  at  new  and  full 

Fig.  108. 


moon ;  and  the  neap  tides  when  the  two  luminaries 
act  in  directions  at  right  angles  to  each  other,  as  is  the 

254.  Explain  the  spring  tides— also  the  neap  tides.    Illustrate  by 
the  figures. 

20* 


234  ASTRONOMY. 

case  when  the  moon  is  in  quadrature.  The  mode  of 
action,  in  each  case,  will  be  clearly  understood  by  in- 
specting Figs.  107  and  108. 

Fig.  107  shows  the  situation  of  the  two  luminaries 
when  they  act  together  at  new  moon.     The  waters  are 
elevated  both  on  the  same  side  of  the  earth  as  the  at- 
tracting  bodies  at  A,  and  also  on  the  opposite  side,  at 
B.     If  we  now  conceive  the  moon  to  change  its  place 
to  B,   when  it  would  be  full  moon,  the  waters  would 
still  have  the  same  elongated  figure  in  the  line  of  the 
two  bodies,  while  at  places  90°  distant,  at  C  and  D,  it^ 
would  be  low  water.     Again,  in  Fig.  108,  the  moon 
being  in  quadrature  at  C,  the  two  attracting  bodies  act ; 
in  opposition  to  each  other,  the  sun  raising  a  tide  at  A 
and  B,  while  the  moon  raises  a  still  higher  tide  at  C  ] 
and  D.     Hence,  the  high  tide  beneath  the  moon,  and 
the  low  tide  at  places  90°  distant,  are  both  less  than 
ordinary. 

255.  The  largest  lakes  and  inland  seas  have  no  per- 
ceptible  tides.  This  is  asserted  by  all  writers  respect- 
ing the  Caspian  and  Black  seas  ;  and  the  same  is  found 
to  be  true  of  the  largest  of  the  North  American  lakes, 
Lake  Superior.  Although  these  several  tracts  of  wa- 
ter appear  large,  when  taken  by  themselves,  yet  they 
occupy  but  small  portions  of  the  surface  of  the  globe, 
as  will  be  evident  on  seeing  how  small  a  space  they 
occupy  on  the  artificial  globe ;  so  that  the  attraction 
of  the  sun  and  moon  is  nearly  equal  on  all  parts  of 
such  sea  or  lake.  But  it  is  the  inequality  of  attraction 
on  different  parts  that  produces  the  tides. 

255.  "Why  have  lakes  and  inland  seas  no  tides  1 


CHAPTER   VI. 
THE  PLANETS. 

•GENERAL  VIEW INFERIOR   PLANETS — SUPERIOR  PLANETS PLANET- 
ARY   MOTIONS.    • 

SECTION  1.  General  View  of  the  Planets. 

256.  THE  name  planet  is  derived  from  a  Greek  word 
which  signifies  a  wanderer,  and  is  applied  to  this  class 
of  bodies,  because  they  shift  their  positions  in  the  heav- 
ens, whereas  the  fixed  stars  constantly  maintain  the 
same  places  with  respect  to  each  other.     The  planets 
known  from  a  high  antiquity   are,   Mercury,   Venus, 
Earth,  Mars,  Jupiter,  and  Saturn.     To  these,  in  1781, 
was  added  Uranus,  (or  Herschel,  as  it  is  sometimes 
called,  from  the  name  of  the  discoverer.)  and,  as  late 
as  the  commencement  of  the  present  century,  four  more 
were  added,  namely,  Ceres,  Pallas,  Juno,  and  Vesta. 
All  these  are  called  primary  planets.     Several  of  them 
have  one  or  more  attendants,  or  satellites,  which  revolve 
around  them,  as  they  revolve  around  the  sun.     The 
Earth  has  one  satellite,  namely,  the  Moon ;  Jupiter  has 
four,  Saturn  seven,  and  Uranus  six.     Mercury,  Venus, 
and  Mars,  are  without  satellites.     The  same  is  the  case 
with  the   four  new  planets,   or  asteroids,  as  they  are 
sometimes   called.     The   whole   number   of    planets, 
therefore,  is  twenty-nine,  namely,  eleven  primary,  and 
eighteen  secondary  planets. 

257.  Mercury  and  Venus  are  called  inferior  planets, 
because  they  have  their  orbits  nearer  to  the  sun  than 
that  of  the  earth  ;  while  all  the  others,  being  more  dis- 
tant from  the  sun  than  the  earth  is,  are  called  superior 

256.  Whence  the  name  planet  1    What  planets  have  been  known, 
from  a  high  antiquity  1    What  have  been  added  to  these  1    What 
is  said  of  the  satellites  1    What  is  the  whole  number  of  planets  1 

257.  Why  are  Mercury  and  Venus  called  inferior  planets  1    Why 
the  others  superior  planets  ^ 


236 


ASTRONOMY. 


planets.     Let  us  now  compare  the  planets  with 
another,  in  regard  to  their  distances  from  the  sun,  the 
magnitudes,  and  their  times  of  revolution. 

258.  Distances  from  the  sun,  in  miles. 


1.  Mercury, 

£ 

37,000,000. 

2.  Venus, 

2 

68,000,000. 

3.  Earth, 

© 

95,000,000. 

4.  Mars, 

c? 

142,000,000. 

5.  Vesta, 

9 

225,000,000. 

6.  Juno, 

0  ) 

7.  Ceres, 

261,000,000. 

8.  Pallas, 

£  3 

9.  Jupiter, 

<2j_ 

485,000,000. 

10.  Saturn, 

T^ 

490,000,000. 

11.  Uranus, 

W 

1800,000,000. 

The  dimensions  of  the  planetary  system  are  seen 
from  this  table  to  be  vast,  comprehending  a  circular 
space  thirty-six  hundred  millions  of  miles  in  diameter. 
A  railway  car,  travelling  constantly  at  the  rate  of 
twenty  miles  an  hour,  would  require  more  than  twenty 
thousand  years  to  cross  the  orbit  of  Uranus. 
259.  Magnitudes. 


1. 

Mercury, 

Diameter. 

3140. 

5. 

Ceres, 

Diameter. 

160 

2. 

Venus, 

7700. 

6. 

Jupiter, 

89,000 

3. 

Earth, 

7912. 

7. 

Saturn, 

79,000 

4. 

Mars, 

4200. 

8. 

Uranus, 

35,000 

We  perceive  that  there  is  a  great  diversity  among 
the  planets,  in  regard  to  size.  While  Venus,  an  infe- 
frior  planet,  is  nine-tenths  as  large  as  the  Earth,  Mars, 
-a  superior  planet,  is  only  one-seventh,  while  Jupiter 
is  twelve  hundred  and  eighty-one  times  as  large.* 

*  The  magnitudes  are  proportioned  to  the  cubes  of  the  diameters. 

258.  Repeat  the  table  of  distances.     "What  is  said  of  the  dimen- 
:sions  of  the  planetary  system  1    How  long  would  a  railway  car  be 
in  crossing  the  orbit  of  Uranus  1 

259.  Repeat  the  table  of  magnitudes.   What  is  said  of  the  diversity 


THE   PLANETS.  237 

Although  several  of  the  planets,  when  nearest  to  us, 
appear  brilliant  and  large  when  compared  with  most 
of  the  fixed  stars,  yet  the  angle  under  which  they  are 
seen  is  very  small,  that  of  Venus,  the  greatest  of  all, 
never  exceeding  about  one  minute,  which  is  less  than 
one  thirtieth  the  apparent  diameter  of  the  sun  or  moon.* 
Jupiter,  also,  by  his  superior  brightness,  sometimes 
makes  a  striking  figure  among  the  stars ;  yet  his  greatest 
apparent  diameter  is  less  than  one  fortieth  that  of  the 
sun. 

260.  Periodic  Times. 


Mercury,  3    months. 

Venus,  7|      " 

Earth,  1    year. 

Mars,  2   years. 


Ceres,       4|  years. 
Jupiter,  12       " 
Saturn,  29 

Uranus,  84 


We  perceive  that  the  planets  nearest  the  sun  move 
most  rapidly.  Mercury  performs  nearly  three  hundred 
and  fifty  revolutions  while  Uranus  performs  one.  The 
apparent  progress  of  the  most  distant  planets  around 
the  sun  is  exceedingly  slow.  Uranus  advances  only  a 
little  more  than  four  degrees  in  a  whole  year ;  so  that 
we  find  this  planet  occupying  the  same  sign,  and  of 
course  remaining  nearly  in  the  same  part  of  the  heavens, 
for  several  years  in  succession. 

SEC.  2.  Of  the  Inferior  Planets. 

261.  Mercury  and  Venus  have  their  orbits  so  far 
within  that  of  the  earth,  that  they  appear  to  us  as 
attendants  upon  the  sun.  Both  planets  appear  either 
in  the  west  a*  little  after  sunset,  or  in  the  east  a  little 

*  In  every  estimation  of  angular  breadths  or  distances,  it  is  convenient  to 
bear  in  mind  that  the  angular  breadth  of  the  sun  or  moon  is  about  half  a 
•degree. 

in  regard  to  size  1    What  of  the  angular  diameter  of  the  planets  1 
How  do  the  largest  compare  with  the  sun  or  moon  1 

260.  Repeat  the  table  of  periodic  times.     What  is  said  of  the 
planets  nearest  the  sun  1    What  of  those  most  distant  1 

261.  How  do  Mercury  and  Venus  appear  with  respect  to  the  sun  1 


238 


ASTRONOMY. 


before  sunrise.  In  high  latitudes,  where  the  twilight  i 
long,  Mercury  can  seldom  be  seen  with  the  naked  eye, 
and  then  only  when  its  angular  distance  from  the  su 
is  greatest.  In  our  latitude,  we  can  usually  catch  a 
glimpse  of  this  planet  for  several  evenings  and  morn- 
ings, if  we  will  watch  the  time  (usually  given  in  the 
almanac)  when  it  is  at  its  greatest  elongations  from  the 
sun.  It,  however,  soon  runs  back  again  to  the  sun. 
The  reason  of  this  will  be  plain  from  the  following 
diagram.  Let  S  represent  the  sun,  E  the  earth, 


E 


M  Q  N  R  the  orbit  of  Mercury,  O  Z  P  an  arc  of  the 
heavens.  Then,  since  we  refer  all  distant  bodies  in  the 
sky  to  the  same  concave  sphere,  we  should  see  the  sun 
at  Z,  in  the  heavens,  and  when  the  planet  was  at  R  or 
Q,  we  should  see  it  close  by  the  sun,  and  when  it  was 

What  of  Mercury  in  high  latitudes  1  What  in  our  latitude  1  When 
do  we  catch  a  glimpse  of  it  1  Explain  the  reason  of  this  from  the 
figure. 


THE   PLANETS.  239 

at  its  greatest  elongation,  at  M  or  N,  we  should  see  it 
at  O  or  P,  when  its  angular  distance  from  the  sun 
would  be  measured  by  the  arc  O  Z  or  P  Z.  Suppose 
Mercury  comes  into  view  at  M,  its  greatest  eastern 
elongation ;  as  it  passes  on  to  Q,  its  inferior  conjunction, 
it  appears  to  move  in  the  sky  backward,  or  contrary  to 
the  order  of  the  signs,  from  O  to  Z  ;  and  it  continues 
its  backward  motion  from  M  to  N,  or  apparently  from 
O  to  P.  But  now  from  N,  its  greatest  western  elongation, 
through  R,  its  superior  conjunction,  to  M,  its  greatest 
eastern  elongation,  its  apparent  motion  is  direct.  Then, 
the  planet  is  said  to  be  in  its  superior  conjunction.  The 
inferior  planets,  Mercury  and  Venus,  appear  to  run 
backward  and  forward  across  the  sun,  Mercury 
receding  so  little  from  that  luminary  as  almost  always 
to  be  lost  in  his  beams.  Venus,  however,  moves  in  a 
larger  orbit,  and  recedes  so  far  from  the  sun,  on  both 
sides,  as  often  to  remain  a  long  time  in  the  evening 
or  morning  sky,  always  immediately  following  or  pre- 
ceding the  sun,  and  hence  called  the  evening  and 
morning  star. 

262.  When  an  inferior  planet  is  near  its  greatest 
•elongation,  on  either  side,  it  presents  to  us,  when  viewed 
with  the  telescope,  half  its  enlightened  disk,  appearing 
to  the  telescope  like  the  moon  in  one  of  her  quarters. 
While  passing  from  the  eastern  to  the  western  elonga- 
tion, through  the  inferior  conjunction,  the  enlightened 
portion  grows  less  and  less,  taking  the  crescent  form, 
like  the  old  of  the  moon,  until  it  arrives  at  the  inferior 
conjunction,  when  it  presents  the  entire  dark  side 
toward  us.  Soon  after  passing  the  conjunction,  it 
appears  like  the  new  moon,  and  increases  to  the  first 
quarter,  at  the  greatest  western  elongation.  When 
passing  through  the  superior  conjunction,  the  other  side 

262.  How  does  an  inferior  planet  appear  when  at  its  greatest 
elongation  1  How  when  between  that  and  the  inferior  conjunction? 
How  toward  the  superior  conjunction  1  In  what  respects  do  they 


240  ASTRONOMY. 

of  the  sun,  the  enlightened  part  constantly  increases, 
and  becomes  like  the  full  moon  in  the  superior  con- 
junction,  after  which  the  enlightened  portion  decreases. 
The  phases  of  Mercury  and  Venus,  therefore,  as  seen 
in  the  telescope,  resemble  the  changes  of  the  moon. 
In  some  respects,  however,  the  appearances  do  not 
correspond  to  those  of  the  moon  ;  for  since,  when  full, 
they  are  in  the  part  of  the  orbit  most  remote  from  us, 
they  appear  then  much  smaller  than  when  on  the  side 
of  the  inferior  conjunction  ;  and  their  nearness  to  the 
sun,  when  full,  also  prevents  their  being  seen  except 
in  the  day  time,  and  then  they  are  invisible  to  the  naked 
eye,  because  their  light  is  lost  in  that  of  the  sun. 
Hence,  these  planets  appear  brightest  when  a  little  lessj 
than  half  their  enlightened  sides  are  turned  toward 


us,  (being  then  just  within  their  greatest  elongation  on^ 
either  side,)  since  their  greater  nearness  to  us  morel 
than  compensates  for  having  in  view  a  less  portion  of; 
the  enlightened  disk,  as  will  be  seen  by  the  acconv 
panying  diagram. 

263.  Mercury  and  Venus  both  revolve  on  their  axes 
in  nearly  the  same  time  with  the  earth,  and  have 
therefore  similar  days  and  nights.  Mercury  owes-' 


resemble  the  changes  of  the  moon  1    How  do  they  differ  1    At  what 
point  do  the  inierior  planets  appear  brightest  1 


THE   PLANETS.  241 

almost  all  its  peculiarities  to  its  nearness  to  the  sun. 
Its  light  and  heat  derived  from  the  sun  are  estimated 
to  be  nearly  seven  times  as  great  as  ours,  and  the  sun 
•would  appear  to  an  inhabitant  of  Mercury  seven  times 
as  large  as  it  does  to  us.  The  motion  of  Mercury,  in  his 
revolution  round  the  sun,  is  swifter  than  that  of  any 
other  planet,  being  more  than  100,000  miles  every 
hour ;  whereas,  that  of  the  Earth  is  less  than  70,000. 
Eighteen  hundred  miles  every  minute — crossing  the 
Atlantic  ocean  in  less  than  two  minutes — this  is  a  ve- 
locity of  which  we  can  form  but  very  inadequate  con- 
ceptions. 

264.  Every  time  Mercury  and  Venus  come  to  their 
inferior  conjunction,  they  would  eclipse  the  sun,  if 
their  orbits  coincided  with  the  earth's  orbit,  or  both 
were  in  the  same  plane ;  as  we  should  have  a  solar 
eclipse  at  every  new  moon,  if  the  moon's  orbit  were 
in  the  same  plane  with  the  earth's.  As,  however,  the 
orbits  of  these  planets  are  inclined  to  the  ecliptic,  they 
are  not  seen  on  the  sun's  disk  except  when  the  con- 
junction takes  place  at  one  of  their  nodes.  They  then 
pass  over  the  sun,  each  in  a  round  black  spot,  and  the 
phenomenon  is  called  a  Transit.  Transits  of  Mer- 
cury and  Venus  occur  but  seldom,  but  are  regarded 
with  the  highest  interest  by  astronomers,  that  of  Ve- 
nus, in  particular ;  for,  by  observing  it  at  distant  points 
on  the  earth,  materials  are  obtained  for  finding  the 
sun's  horizontal  parallax,  which  enables  astronomers 
to  calculate  the  distance  of  the  sun  from  the  earth. 
(See  Art.  219.)  In  the  transits  of  Venus,  in  1761 
and  1769,  several  European  governments  fitted  out 
expensive  expeditions  to  parts  of  the  earth  remote 
from  each  other.  For  this  purpose,  the  celebrated 

263.  In  what  time  do  Mercury  and  Venus  revolve  on  their  axesl 
To  what  does  Mercury  owe  its  peculiarities  1    Explain  his  swiftness 
of  motion. 

264.  Why  do  not  Mercury  and  Venus  eclipse  the  sun  at  every  in- 
ferior conjunction  1  What  is  a  transit  1  Why  regarded  with  so  great 

21 


242 


ASTRONOMY. 


Captain  Cook,  in  1769,  went  to  the  South  Pacific 
Ocean,  and  observed  the  transit  of  Venus  at  the  island 
of  Otaheite,  (Tahiti,)  while  others  went  to  Lapland 
for  the  same  purpose,  and  others,  still,  to  many  other 
parts  of  the  globe.  The  next  transit  of  Venus  will 
happen  in  1874. 

SEC.  3.   Of  the  Superior  Planets. 

265.  All  .the  planets,  except  Mercury  and  Venus, 
have  their  orbits  farther  from  the  sun  than  the  earth's 
orbit.  They  are  seen  in  superior  conjunction  with 
the  sun,  and  in  opposition,  like  the  moon  when  full ; 
but  as  they  are  always  more  distant  from  the  sun  than 


the  earth  is,  they  can  never  come  into  inferior  con- 
junction.    This  will  be  plain  from  the  foregoing  dia- 

interest  1    What  is  said  of  the  transits  of  Venus  in  1761  and  176ft 
When  will  the  next  transit  of  Venus  happen  1 


THE   PLANETS.  -  243 

gram.  Let  the  Earth  be  at  E,  and  a  superior  planet, 
as  Mars,  in  different  parts  of  his  orbit,  M  Q,  M'.  At 
M',  the  planet  would  be  seen  in  the  same  part  of  the 
heavens  with  the  sun,  rising  and  setting  at  the  same 
time  with  him,  and  would  therefore  be  in  conjunction; 
but  being  the  other  side  of  the  sun,  it  would,  of  course, 
be  a  superior  conjunction.  At  Q,  the  planet  would  ap- 
pear in  quadrature,  and  at  M,  in  opposition,  rising  when 
the  sun  sets,  like  the  full  moon. 

266.  The  superior  planets,  however,  do  not,  like  the 
inferior,  undergo  the  same  changes  as  the  moon,  but, 
with  the  exception  of  Mars,  always  present  to  the 
telescope  their  disks  fully  enlightened  ;  for,  if  we 
viewed  them  from  the  sun,  we  should  have  the  whole 
enlightened  side  turned  constantly  toward  us;  and 
so  small  is  our  own  distance  from  the  sun,  compared 
with  that  of  Jupiter,  Saturn,  or  Uranus,  that  we  view 
them  nearly  as  though  we  stood  on  the  sun.  Mars, 
being  nearer  the  earth,  does  in  fact  change  his  figure 
slightly  ;  for,  when  seen  in  quadrature,  at  Q,  a  small 
part  of  the  enlightened  hemisphere  is  concealed  from 
us,  and  the  planet  appears  gibbous,  like  the  moon 
when  a  little  past  the  full.  The  superior  planets, 
however,  undergo  considerable  changes  in  apparent 
magnitude  and  brightness,  being  at  one  time  much 
nearer  to  us  than  at  another.  Thus,  in  Fig.  Ill, 
Mars,  when  at  M,  in  opposition,  is  nearer  the  Earth 
than  at  M',  in  superior  conjunction,  by  the  whole 
diameter  of  the  earth's  orbit — a  space  of  about 
190,000,000  miles.  Hence,  when  this  planet  is  in 
opposition,  rising  soon  after  the  sun  sets,  it  often  sur- 
prises us  by  its  unusual  splendor,  which  appears  more 

265.  What  are  superior  planets'?    How  do  they  differ  from  the 
inferior  ?     Explain  their  conjunction  and  opposition  by  the  tigure. 

266.  Have  the  superior  planets  any  phases  1     What  is  said  of  the 
phases  of  Mars  1     what  changes  of  apparent  magnitude  do  the  su- 
perior planets  undergo  1    Explain  the  cause  of  these. 


244  ASTRONOMY. 

striking  on  account  of  its  fiery  red  color.  All  the  other 
planets,  likewise,  appear  finest  when  in  opposition,  al- 
though the  remoter  planets  are  less  altered  than  those 
that  are  nearer  to  us. 

267.  JUPITER  is  distinguished  from  all  the  other 
planets  by  his  great  magnitude.  His  diameter  is 
89,000  miles,  and  his  volume  1281  times  that  of  the 
earth.  He  revolves  on  his  axis  once  in  about  ten 
hours,  giving  to  places  near  his  equator  a  motion 


112. 


twenty-seven  times  as  swift  as  on  the  earth.  It  will 
be  recollected,  also,  that  the  distance  of  Jupiter  from 
the  sun  is  485,000,000  miles,  and  that  his  revolution 
around  the  sun  occupies  twelve  years ;  so  that  every 
thing  belonging  to  this  planet  is  on  a  grand  scale. 
The  view  of  Jupiter  through  a  good  telescope,  is  one 
of  the  most  splendid  and  interesting  sights  in  astrono- 
my. The  disk  expands  into  a  large  and  bright  orb, 
like  the  full  moon  ;  across  the  disk,  arranged  in  paral- 
lel stripes,  are  several  dusky  bands,  called  belts  ;  and 

267.  By  what  is  Jupiter  distinguished  from  all  the  other  planets'' 

Lis  diameter— volume— distance  from  the  sun  1    View  of  Jupiter 

through  a  good  telescope  1    Appearance  of  his  disk,  belts,  and  sat 


THE   PLANETS.  245 

four  bright  satellites,  or  moons,  constantly  varying 
their  positions,  add  another  feature  of  peculiar  mag- 
nificence* 

268,  SATURN  has  also  within  itself  a  system  full  of 
grandeur.  Next  to  Jupiter,  it  is  the  largest  of  the 
planets,  being  79,000  miles  in  diameter,  or  about  1000 
times  as  large  as  the  earth.  It  has,  likewise,  belts  on 
its  surface,  though  less  distinct  than  those  of  Jupiter. 


Fig.  113. 


But  the  great  peculiarity  of  Saturn  is  its  Ring,  a  broad 
wheel,  encompassing  the  planet  at  a  great  distance  from 
it.  What  appears  to  be  a  single  ring,  when  viewed 
with  a  small  telescope,  is  found,  when  examined  by 
powerful  telescopes,  to  consist  of  two  rings,  separated 
from  each  other  by  a  dark  line  of  the  sky,  seen  between 
them.  Although  the  division  of  the  rings  appears  to 
us,  on  account  of  our  immense  distance,  as  only  a  fine 
line,  yet  it  is  in  reality  an  interval  of  not  less  than 
1,800  miles ;  and,  although  we  see  in  the  telescope 
but  a. small  speck  of  sky  between  the  planet  and  the 
ring,  yet  it  is  really  a  space  20,000  miles  broad.  The 

268.  Saturn  compared  with  Jupiter — diameter — vplume— belts — 
Ring-* what  is  said  of  this  1    Distance  between  the  rings.    Breadth 
21* 


246 


ASTRONOMY. 


breadth  of  the  inner  wheel  is  17,000  miles,  and  that 
of  the  outer,  10,500  miles  ;  so  that  the  entire  diameter 
of  the  outer  ring,  from  outside  to  outside,  is  179,000 
miles.  These  rings  are  so  far  from  the  body  of  the 
planet,  that  an  inhabitant  of  that  world  would  not  take 
them  for  appendages  to  his  own  planet,  but  would  view 
them  as  magnificent  arches  on  the  face  of  the  starry 
heavens. 

Fig.  114. 


269.  Saturn's  ring,  in  its  revolution  with  the  planet 
around  the  sun  once  in  about  thirty  years,  always  keeps 
parallel  to  itself,  as  is  represented  in  the  annexed 
diagram,  where  the  small  circle,  a  Z>,  is  the  earth's 
orbit,  and  Saturn  is  exhibited  in  eight  different  positions 
in  his  orbit.  If  we  hold  a  circle,  as  a  piece  of  coin, 
directly  before  the  eye,  we  see  the  entire  circle  ;  but 
if  we  hold  it  obliquely,  it  appears  an  ellipse  ;  and  if 
we  turn  it  round  until  we  see  it  edgewise,  the  ellipse 
grows  constantly  narrower  and  narrower,  until,  when 
the  edge  is  toward  us,  we  see  nothing  but  a  line.  If 

of  each  wheel.     Entire  diameter  of  the  outer  ring.    What  is  said 
of  the  appearance  of  the  rings  from  the  planet  1 

269.  What  position  does  the  ring  keep  in  its  revolution  around  the 
sun  1  Describe  Fig  114.  Into  what  figures  is  a  circle  projected 


THE    PLANETS.  247 

the  learner  obtains  a  clear  idea  of  these  appearances, 
he  will  easily  understand  the  different  appearances  of 
Saturn's  ring.  In  two  points  of  the  revolution  around 
the  sun,  at  A  and  E,  the  edge  is  presented  to  us,  and 
we  see  the  ring  only  as  a  fine  line,  or,  perhaps,  lose 
sight  of  it  altogether.  After  passing  this  point,  from 
B  to  C,  we  see  more  and  more  of  the  ellipse,  until,  in 
about  seven  years,  it  arrives  at  C,  when  it  appears  quite 
broad,  as  represented  in  figure  114.  Then  it  gradually 
closes  again  for  seven  years  more,  and  dwindles  into  a 
line  at  E. 

270.  Saturn  is  attended  by  seven  satellites.     Al- 
though they  are  bodies  of  considerable  size,  yet,  on 
account  of  their  immense  distance  from  us,  they  appear 
exceedingly  minute,  and  require  superior  telescopes 
to  see  them  at  all.     It  is  accounted  a  good  telescope 
which  will  give  a  distinct  view  of  even  three  of  the 
•  satellites  of  Saturn,  and  the  whole  seven  can  be  seen 
only  by  the  most  powerful  telescopes  in  the  world. 

271.  URANUS  is  also  a  large  body,  being  35,000 
miles  in  diameter ;  but  being  1800,000,000  miles  off, 
it  is  scarcely  seen  except  by  the  telescope,  and  would 
hardly  be  distinguished  from  a  fixed  star,  if  it  were 
not  seen  to  have  the  motions  of  a  planet.     In  the  most 
powerful  telescopes,  however,  it  exhibits  more  of  the 
character  of  a  planet.     Herschel  saw,  as  he  supposed, 
six  satellites  belonging  to  this  planet,  but  only  two  are 
commonly  visible  to  the  best  telescopes.     So  distant  is 
this  planet,  that  the  sun  himself  would  appear  from  it 
400  times  less  than  he  does  to  us,  and  it  receives  from 
him  light  and  heat  proportionally  feeble. 

when  seen  in  different  positions  ^  In  what  points  is  the  edge  pre- 
sented to  us  1  When  does  it  appear  broadest  1 

270.  How  many  satellites  has  Saturn  1    How  do  they  appear  to 
the  telescope  1    What  power  does  it  require  to  see  them  1 

271.  Uranus— his  diameter— distance  from  the  sun— appearance 
in  the  telescope— number  of  satellites.    How  would  the  sun  appear 
from  Uranus  ] 


248  ASTRONOMY. 


272.  The    NEtv    PLANETS,   or    ASTEROIDS,   Ceres, 
Pallas,    Juno,   and    Vesta,    were    unknown   until    the 
commencement  of  the  present  century.     They  are  so 
small  as  to  be  invisible  to  the  naked  eye,  but  are  seen  by 
telescopes  of  moderate  power.    They  lie  near  together 
in  the  large  space  between  the  orbits  of  Mars  and 
Jupiter,  at  an  average  distance  from  the  sun  of  about 
250,000,000  miles. 

SEC.  4.  Of  the  Planetary  Motions. 

273.  The  planets  all  revolve  around  the  sun  in  the 
same  direction,  from  west  to  east,  and  pursue  nearly 
the   same  path  in  the  heavens.      Mercury  wanders 
farthest  from  the  general  track,  but  he  is  never  seen 
farther  than  about  seven  degrees  from  the  ecliptic.    The 
others,  with  the  exception  of  the  Asteroids,  are  always 
seen  close  in  the  neighborhood  of  the  ecliptic,  and  we 
never  need  to  look  in  any  other  part  of  the  sky  for  a 
planet,  than  in  the  region  of  the  sun's  apparent  path  in 
the  heavens. 

274.  If  we  could  stand  on  the  sun  and  view  the* 
planets  move  round  it,  their  motions  would  appear 
very  simple.     We  should  see  them,  one  after  another, 
pursuing  their  way  along  the  great  highway  of  the 
heavens,  the   zodiac,   rolling   around  the  sun  as  the 
moon  does  around  the  earth,  though  with  very  different 
degrees  of  speed,   those  near  the  sun    moving  with 
far  more  rapidity  than  those  more  remote,  often  over- 
taking  them,  and  passing  rapidly  by  them.     Mercury, 
especially,  comes  up  with  and  passes  Jupiter,  Saturn, 
atid  Uranus,  a  great  number  of  times  while  they  are 

272.  What  is  said  of  the  New  Planets  —  their  discovery  —  size  — 
position  in  the  solar  system  —  distance  from  the  sum  1 

273.  Pknetary  motions  —  through  what  part  of  the  heavens—  which 
•wanders  farthest  from  the  ecliptic  1 

274.  If  we  could  view  the  planets  from  the  sun,  how  would  they 
appear  to  move  1    In  what  orbits,  and  with  what  difierciit  degrees 
oi  speed  1 


THE  PLANETS.  249 

making  their  tardy  circuit  around  the  sun.  To  a  spec- 
tator thus  situated,  the  planets  would  all  appear  to 
move  around  him  in  great  circles,  such  being  their 
projections  on  the  face  of  the  sky.  They  are,  how- 
ever, not  perfect  circles,  but  are  a  little  shorter  in  one 
direction  than  the  other,  forming  an  oval  or  ellipse. 

275.  Such  would  be  the  appearances  of  the  planet- 
ary motions  if  viewed  from  the  center  of  their  motions, 
that  is,  at  the  sun,  and  such  they  are  in  fact.     But  two 
causes  operate  to  make  the  motions  of  the  planets  ap- 
pear very  different  from  what  they  really  are ;  first,  we 
view  them  out  of  the  center  of  their  motions,  and,  sec- 
ondly, we  are  ourselves  in  motion.     We  have  seen, 
in  the  case  of  the  inferior  planets,  Mercury  and  Ve- 
nus, that  our  being  out  of  the  center  makes  them  ap- 
pear to  run  backward  and  forward   across  the   sun, 
although  they  are  all  the  while  moving  steadily  on  in 
one  direction;    and  we  know  that  our  own   motion 
along  with  the  earth  on  its  axis,  every  day,  makes  the 
heavens   appear  to   move   in   the   opposite   direction. 
Hence,  we  see  how  very  different  may  be  the  actual 
motions  of  the  planets  from  what  they  appear  to  be. 
As  we  have  said,  they  are  actually  very  simple,  mov- 
ing steadily  round  the  sun,  all  in  one  direction ;  but 
their   apparent    motions   are    exceedingly    irregular. 
They  sometimes  move  faster  and  sometimes  slower 
— backward  and  forward — and  at  times  appear  to  stand 
still  for  a  considerable  period. 

276.  If  we  have  ever  passed  swiftly  by  a  small  ves- 
sel, sailing  in  the  same  direction  with  ourselves,  but 
much  slower,  we  may  have  seen  the  vessel  appear  to 
be  moving  backward,  stern  foremost.     For  a  similar 
reason,  the  superior  planets  sometimes  seem  to  move 
backward,  merely  because  the  earth  has   a   swifter 

275.  What  makes  the  planetary  motions  appear  very  different 
from  what  they  really  are  1  Are  the  real  motions  more  or  less  sim- 
ple than  the  apparent  1 


250 


ASTRONOMY. 


motion,  and  sails  rapidly  by  them.  Then  again  they 
seem  to  stand  still,  because  they  are  about  turning, 
when  our  motion  has  ceased  to  carry  them  apparently 
backward  any  farther,  and  they  are  recovering  their 
direct  motion.  They  appear  also  to  stand  still,  when 
they  are  moving  directly  toward  us  or  from  us,  as 
Mercury  or  Venus  does  when  near  its  greatest  elonga- 
tion. (See  Fig.  109,  page  233.)  A  diagram  will  as. 
sist  us  in  obtaining  a  clear  idea  of  the  way  in  which 
these  appearances  are  produced. 

Fig.  115. 


277.  Let  the  inner  circle,  ABC,  represent  the 
earth's  orbit,  and  the  outer  circle  the  orbit,  of  Mars, 


276.  Appearance  of  a  vessel  when  we  pass  rapidly  by  it  1  Why  d 
the  superior  planets  appear  to  move  backward,  and  to  stand  still  1 


THE  PLANETS.  251 


(or  any  other  superior  planet,)  and  N  R  a  portion  of 
the  concave  sphere  of  the  heavens.  To  make  the 
case  simple,  we  will  suppose  Mars  to  be  stationary  at 
M,  in  opposition  ;  for,  although  he  is  actually  moving 
eastward  all  the  while,  yet,  since  the  earth  is  moving 
the  same  way  more  rapidly,  their  relative  situations 
will  be  the  same,  if  we  suppose  Mars  to  stand  still  and 
the  earth  to  move  on  with  the  excess  of  its  motion 
above  that  of  the  planet.  As  the  earth  moves  from  A 
to  B,  Mars  appears  to  move  backward  from  P  to  N  ; 
for  the  planet  will  always  appear  in  the  heavens  in  the 
direction  of  the  straight  line,  as  B  M,  drawn  from  the 
spectator  to  the  body.  When  the  earth  is  at  B,  Mars 
appears  stationary,  because  the  earth  is  moving  directly 
from  him,  and  the  line  B  M  N  does  not  change  its  di- 
rection. But  while  the  earth  moves  on  to  C,  D,  E, 
F,  the  planet  resumes  a  direct  motion  eastward  through 
O,  F,  Q,  R.  Here  it  again  stands  still,  while  the 
earth  is  moving  directly  toward  it,  and  then  goes  back- 
ward again.  When  the  planet  is  in  opposition,  the 
earth  being  at  A,  its  motion  appears  more  rapid  than 
in  other  situations,  because  then  it  is  nearest  to  us. 
In  the  superior  conjunction,  when  the  earth  is  at  D, 
the  motion  of  Mars  is  comparatively  slow. 

278.  There  are  three  great  Laws  that  regulate  the 
motions  of  all  bodies  belonging  to  the  Solar  System, 
called  KEPLER'S  Laws,  from  the  name  of  the  great  as- 
tronomer who  discovered  them.  The  first  is,  that  the 
orbits  of  the  earth  and  all  the  planets  are  ellipses,  having 
the  sun  in  one  of  the  foci  of  the  ellipse.  Figure  116  re- 
presents such  an  ellipse,  differing  but  little  from  a  cir- 
cle, but  still  having  the  diameter,  A  B,  called  the  major 
axis  of  the  orbit,  perceptibly  longer  than  C  D.  The 


277.  Illustrate  the  motion  of  Mars  from  Fig,U§. 
tion  most  rapid  1    When  slow  1 

278.  Kepler's  Laws.    Repeat  the  first  law.    What  is  an  eUipsfc— 
the  major  axis—  foci—  perihelion—  aphelion  1 


ASTRONOMY. 


two  points,  E  and  F,  (being  the  points  from  which,  by 
a  certain  process,  the  figure  is  described,)  are  called 


the  two  foci,  and  each  of  them,  a  focus,  of  the  ellipse. 
Suppose  the  sun  at  F,  then  B  will  be  the  perihelion  or 
nearest  distance  of  a  planet  to  the  sun,  and  A  is  the 
aphelion,  or  farthest  distance. 

279.  A  line  drawn  from  the  sun  to  a  planet  is  called 
the  radius  vector,  as  E  a  or  E  #,  (Fig.  117 ;)  and  the 
second  of  Kepler's  Laws  is,  that  while  a  planet  is  going 
round  the  sun,  the  radius  vector  passes  over  equal  spaces 
in  equal  times.  The  meaning  of  this  is,  that,  if  an 
imaginary  line,  as  a  cord,  were  extended  from  the  sun 
to  any  planet,  this  cord  would  sweep  over  just  as  much 
space  one  day  as  another.  When  the  planet  is  at  its 
perihelion,  the  cord  would,  indeed,  move  faster  than 
toward  the  aphelion ;  but  it  would  also  be  shorter, 


279.  What  is  the  radius  vector  1    Repeat  the  second  law. 
plain  its  meaning. 


Ex- 


THE    PLANETS. 


253 


and  the  greater  breadth  of  the  space,  E  a  J,  would 
make  it  just  equal  to  the  narrower  but  longer  space, 

Fig.  117. 


E  I  m.     This  law  has  been  of  incalculable  service  in 
all  the  higher  investigations  of  astronomy. 

280.  The  third  of  Kepler's  Laws  is,  that  the  squares 
of  the  periodic  times  of  different  planets,  are  proportioned 
to  the  cubes  of  the  major  axes  of  their  orbits.  Now  the 
periodic  time  of  a  planet,  or  the  time  it  takes  to  go  round 
the  sun,  from  any  star  back  to  the  same  star  again,  can 
be  seen  by  watching  it,  as  has  often  been  done,  during 
the  whole  of  its  revolution.  We  also  know  the  length 
of  the  major  axis  of  the  earth's  orbit,  because  it  is  just 
twice  the  average  or  mean  distance  of  the  earth  from 
the  sun.  These  things  being  known,  we  can  find  the 
distance  of  any  of  the  planets  from  the  sun  by  a  simple 
statement  in  the  rule  of  three.  For  example,  let  it  be 
required  to  find  the  major  axis  of  Jupiter's  orbit,  or  the 

280.  Repeat  the  third  law.    What  is  meant  by  the  periodic  time 
of  a  planet  1    How  may  the  periodic  time  be  found  J    Do  we  know 
22 


254  ASTRONOMY. 

mean  distance  of  Jupiter  from  the  sun,  which  is  half 
the  length  of  that  axis.  Then,  since  the  earth's 
periodic  time  is  one  year,  and  Jupiter's  twelve  years, 
(putting  E  for  the  earth's  distance  from  the  sun,  and  J 
for  Jupiter's,)  we  say, 

I2  :  122  :  :  E3  :  J3. 

Now  the  three  first  terms  in  this  proportion  are 
known,  and  hence  we  can  find  the  fourth,  which  is 
the  cube  of  Jupiter's  distance  from  the  sun  ;  and,  on 
extracting  the  cube  root,  we  find  the  distance  itself. 
We  see,  therefore,  that  the  planetary  system  is  laid  off 
by  an  exact  mathematical  scale. 

281.  The  three  foregoing  laws  are  so  many  great 
facts,  fully  entitled  to  be  called  general  principles, 
because  they  are  applicable  not  only  to  this  or  that 
planet,  but  to  all  the  planets  alike,  and  even  to  comets, 
and  every  other  kind  of  body  that  may  chance  to  be 
discovered  in  the  solar  system.  They  are  the  rules 
according  to  which  all  the  motions  of  the  system  are 
performed.  But  there  is  a  still  higher  inquiry,  respect- 
ing the  causes  of  the  planetary  motions,  which  aims  at 
ascertaining  not  in  what  manner  the  planets  move,  but 
why  they  move  at  all,  and  by  what  forces  their  motions 
are  produced  and  sustained.  Sir  Isaac  Newton  first 
discovered  the  great  principle  upon  which  all  the 
motions  of  the  heavenly  bodies  depend,  that  of  Universal 
Gravitation.  In  its  simplest  expression  it  is  nearly  this : 
all  matter  attracts  all  other  matter.  But  a  more  precise 
expression  of  the  law  of  gravitation  is  as  follows  : 
Every  body  in  the  universe,  whether  great  or  small,  attracts 
every  other  body,  with  a  force  which  is  proportioned  to 

the  major  axis  of  the  earth's  orbit  1  How  to  find  the  major  axis  oi 
Jupiter's  orbit  1 

281.  Why  are  these  laws  called  general  principles  1  What  higher 
inquiry  is  there  1  Who  first  discovered  the  grand  law  of  the  celes- 
tial motions'?  What  is  it  called  1  Its  simplest  expression.  Ita 
more  precise  expression 


THE   PLANETS.  255 

the  quantity  of  matter  directly,  and  to  the  square  of  the 
distance  inversely. 

282.  This  is  the  most  comprehensive  and  important 
of  all  the  laws  of  nature,  and  ought  therefore  to  be 
clearly  understood  in.  its  several  parts.  First,  it  asserts 
that  all  matter  in  the  universe  is  subject  to  it.  In  this 
respect  it  differs  from  Gravity,  which  respects  only  the 
attraction  exerted  by  the  earth,  and  all  bodies  within 
the  sphere  of  its  influence.  But  Universal  Gravitation 
embraces  the  whole  solar  system — sun,  moon,  planets, 
comets,  and  any  other  form  of  matter  within  the 
system.  Nor  does  it  stop  here  ;  it  extends  likewise  to 
the  stars,  and  comprehends  the  infinitude  of  worlds 
that  lie  in  boundless  space.  Secondly,  the  law  asserts 
that  the  attraction  of  gravitation  is  in  proportion  to  the 
quantity  of  matter.  Every  body  gives  and  receives  of  this 
mysterious  influence  an  amount  exactly  proportioned 
to  its  weight ;  and  hence  all  bodies  exert  an  equal 
force  on  each  other.  The  sun  attracts  the  earth  and 
the  earth  the  sun,  and  one  just  as  much  as  the  other  ; 
for  if  the  sun,  in  consequence  of  its  having  354,000 
times  as  much  matter  as  the  earth,  exerts  upon  it 
354,000  times  as  much  force  as  it  would  do  if  it  had 
the  same  weight  with  the  earth,  it  also  receives  from 
the  earth  so  much  more  in  consequence  of  its  greater 
weight.  Were  the  sun  divided  into  354,000  bodies, 
each  as  heavy  as  the  earth,  every  one  would  receive 
an  equal  share  of  the  earth's  attraction,  and  of 
course  the  whole  would  receive  in  the  same  degree 
as  they  imparted.  Thirdly,  the  law  asserts  that,  at 
different  distances,  the  force  of  gravitation  is  in- 
versely  as  the  square  of  the  distance.  If  a  body 
is  twice  as  far  off,  it  attracts  and  is  attracted  four 

282.  What  is  said  of  the  importance  of  this  law  1  What  does  it 
assert  first — what  secondly  ?  How  much  does  every  body  give  and 
receive  of  this  influence  7  Example  in  the  earth  and  sun.  What 
does  the  law  assert  thirdly  ?  How  much  less  does  a  body  attract 
another  when  twice  as  far  off,  or  ten  times  as  far  1 


256  ASTRONOMY. 

times  less  ;  if  ten  times  as  far,  one  hundred  times 
less  ;  if  a  hundred  times  as  far,  ten  thousand  times 
less. 

283.  This   great    principle,   which    has    led   to   a 
knowledge  of  the  causes  of  the  celestial  motions,  and 
given  us  an  insight  into  the  machinery  of  the  Universe, 
was  discovered  by  Sir  Isaac  Newton,  who  is  generally 
acknowledged  to  have  had  the  most  profound  mind  of 
any  philosopher  that  has  ever  lived.     He  was  born  in 
a  country  town  in  England  in  the  year  1642.     He  was 
a  farmer's  son,  and  his  father  having  died  before  he 
was  born,  his  friends  designed  him  for  a  farmer ;  but 
his  strong  and  unconquerable  passion  for  study,  and  the 
great  mechanical  genius  he  displayed  in  his  boyhood, 
led  them  to  the  fortunate  determination  to  educate  him 
at  the  University. 

284.  But  let  us  see  how  the  principle  of  Gravitation 
is  applied  to  explain  the  revolutions  of  the  heavenly 
bodies.     If  I  throw  a  stone  horizontally,  the  attraction 
of  the  earth  will  continually  draw  it  downwards,  out  of 
the  line  of  direction  in  which  it  was  thrown,  and  make 
it  descend  to  the  earth  in  a  curve.     The  particular 
form  of  the  curve  will  depend  on  the  velocity  with 
which  it  is  thrown.     It  will  always  "begin  to  move  in 
the  line  of  direction  in  which  it  is  projected ;  but  it  will 
soon  be  turned  from  that  line  toward  the  earth.    It  will, 
however,   continue  nearer  to  the   line  of  projection, 
in  proportion  as  the  velocity  of  projection  is  greater. 
Let  A  C  (Fig.   118)   be  perpendicular  to  the  hori- 
zon,  and   A   B    parallel   to   it,    and   let   a   stone    be 
thrown  from  A  in  the  direction  of  A  B.      It  will,  in 
every  case,   commence  its  motion  in  the  line  A  B, 
which  will  therefore  be  a  tangent  to  the  curve  it  de- 


283.  What  is  said  of  Sir  Isaac  Newton  1 

284.  How  is  the  principle  of  universal  gravitation  applied  to  the 
explanation  of  the  celestial  motions  1     How  will  a  stone  move 
when  thrown  horizontally  1    Explain  Fig.  118. 


THE  PLANETS. 


257 


scribes ;  but,  if  it  be  thrown  with  a  small  velocity,  it 
will  soon  depart  from  the  tangent,  describing  the  curve 

Fig.  118. 


A  D  ;  with  a  greater  velocity,  it  will  describe  a  curve 
nearer  the  tangent,  at  A  E ;  and  with  a  still  greater 
velocity,  it  will  describe  the  curve  A  F. 

285.  As  an  example  of  a  body  revolving   in   an 
orbit  under  the  influence  of  two  forces,  suppose  a  body 


Fig.  119. 


placed  at  any  point,  P,  (Fig.  119,)  above  the  surface 
of  the  earth,  and  let  P  A  be  the  direction  of  the  earth's 


285.  Explain  the  motions  of  a  body  from  Fig.  119. 
22* 


258 


ASTRONOMY. 


center,  or  a  line  perpendicular  to  the  horizon.     If  the 
body   were  allowed  to  move,  without   receiving  any 
impulse,  it  would  descend  to  the  earth  in  the  direction 
of  P  A  with  an  accelerated  motion.     But  suppose  that 
at  the  moment  of  its  departure  from  P,  it  receives  a 
blow  in  the  direction  P  B,  which  would  carry  it  to  B  !i 
in  the  time  the  body  would  fall  from  P  to  A ;  then  i 
under  the  influence  of  both  forces,  it  would  descend 
along  the  curve  P  D.     If  a  stronger  blow  were  given 
to  it  in  the  direction  P  B,  it  would  describe  a  larger 
curve,  P  E  ;  or,  finally,  if  the  impulse  were  sufficiently  \ 
strong,  it  would  circulate  quite  round  the  earth,  de-  i 
scribing  the  circle  P  F  G.     With  a  velocity  of  projec- 
tion still  greater,  it  would  describe  an  ellipse,  P  I  K ; 
and  if  the  velocity  were  increased  to  a  certain  degree, 
the  figure  would  become  a  parabola,  L  P  M, — a  curve 
which  never  returns  into  itself. 

286.  Now  let  us  con- 
sider   the   same    princi- 
ples in  reference  to  the 
motion  of  a  planet  around 
the   sun.      Suppose   the 
planet   to    have    passed 
\Kthe  point  C,  (Fig.  120,); 
at  the  aphelion,  with  so;j 
small  a  velocity,  that  the 
attraction     of    the    sunj 
bends   its    path    toward' 
itself.     As  the  body  ap- ' 
proaches  the  sun,  since 
the  sun's  attractive  force  • 
is  rapidly   increased  as 
the   distance    is    dimin- 1 
ished,  the  planet's  motion  is  continually  accelerated,] 
and  becomes  very  swift  as  it  approaches  nearer  the  sun.  j 
But,  when  a  body  is  revolving  in  a  curve,  an  increase  of  1 

286.  Explain  the  motions  of  a  planet  from  Fig.  120. 


THE  PLANETS.  259 

velocity  causes  a  rapid  increase  in  the  centrifugal  force, 
and  makes  it  endeavor  with  more  and  more  force  to  fly 
off  in  the  direction  of  a  tangent  to  its  orbit.  Hence,  the 
increase  of  velocity  as  it  approaches  the  sun,  will  not 
carry  it  into  the  sun,  but  the  more  rapid  increase  of 
the  centrifugal  force  will  keep  it  off,  and  carry  it  by, 
and  finally  make  it  describe  the  remaining  portion  of 
the  curve,  back  to  the  place  where  it  set  out.  After 
it  passes  the  perihelion,  at  G,  the  sun's  attraction  con- 
stantly operates  to  hold  it  back,  and  as  it  proceeds 
through  H  and  K  to  A  and  C,  it  is  like  a, ball  rolled 
up  hill,  until  finally  its  motion  becomes  so  slow,  that 
the  centrifugal  force  yields  to  the  force  of  attraction, 
and  it  turns  about  to  renew  the  same  circuit. 

287.  Since  the  nature  of  the  curve  which  any  planet 
describes  depends  on  the  proportion  between  the  two 
forces,  of  projection  and  attraction,  astronomers  have 
inquired  what  proportion  must  have  been  observed 
when  the  planets  were  first  launched  into  space,  in 
order  that  they  should  have  revolved  in  the  orbits  they 
have ;  and  it  is  found  that  the  forces  were  so  adjusted 
as  to  make  the  centrifugal  and  attractive  forces  nearly 
equal,  that  of  projection  being  a  little  greater.  Had 
they  been  exactly  equal,  the  curve  would  have  been  a 
circle ;  and  had  the  force  of  projection  been  much 
greater  than  it  was,  the  ellipses  would  have  been  much 
longer,  and  the  whole  system  much  more  irregular. 
The  planets  also  revolve  on  their  axes  at  the  same  time 
that  they  revolve  around  the  sun  ;  and  astronomers 
have  inquired  what  must  have  been  the  nature  of  the 
impulses  originally  given,  in  order  to  have  produced 
these  two  motions  such  as  they  are.  If  we  strike  a 
ball  in  the  exact  line  of  the  center  of  gravity,  it  will 
move  forward  without  turning  on  its  axis ;  but  if  we 


287.  How  were  the  forces  of  projection  and  attraction  adjusted  to 
each  other,  when  the  planets  were  first  launched  into  space  1  How 


260  ASTRONOMY. 

strike  it  out  of  that  direction  we  can  make  it  move  for- 
ward and  turn  on  its  axis  at  the  same  time.  It  is  cal- 
culated that  the  earth  must  have  received  the  impulse 
which  gave  to  her  her  two  motions,  at  a  distance  from 
the  center  equal  to  the  T|^th  part  of  the  earth's  radius. 
Such  an  impulse  would  suffice  to  give  the  two  motions 
in  question  ;  but  it  would  be  presumptuous  to  under- 
take to  assign  the  exact  mode  by  which  the  Almighty 
first  impressed  upon  the  planetary  system  its  harmoni- 
ous movements;  and  all  such  expressions  as  "  launch- 
ing these  bodies  into  space,"  or  "  impelling"  them  in 
certain  directions,  must  be  regarded  as  mere  figures  of 
speech. 

288.  Besides  explaining  the  revolutions  of  the  hea- 
venly bodies,  the  principle  of  universal  gravitation  ac- 
counts for  all  their  irregularities.  Since  every  body 
in  the  solar  system  attracts  every  other,  each  is  liable 
to  be  drawn  out  of  its  customary  path,  and  all  the 
bodies  tend  mutually  to  disturb  each  other's  motions. 
Most  of  them  are  so  far  apart  as  to  feel  each  other's 
influence  but  little  ;  but  in  other  cases,  where  any  two 
bodies  come  far  within  each  other's  sphere  of  attrac- 
tion, the  mutual  disturbance  of  their  motions  is  very 
great.  The  moon,  especially,  has  its  motions  con- 
tinually disturbed  by  the  attractive  force  of  the  sun. 
When  the  sun  acts  equally  on  the  earth  and  the  moon, 
as  it  does  when  the  two  bodies  are  at  the  same  dis- 
tance from  him,  he  does  not  disturb  their  mutual  rela- 
tions ;  as  the  passengers  on  board  a  steamboat  main- 
tain the  same  position  with  respect  to  each  other, 
whether  the  boat  is  going  with  or  against  the  current. 
But,  at  new  moon,  the  moon  being  nearer  the  sun  than 

must  they  have  been  impelled  in  order  to  have  the  two  motions  1 
How  must  the  earth  have  been  struck  1 

288  Besides  the  revolutions  of  the  heavenly  bodies,  for  what  else 
does  the  principle  of  universal  gravitation  account  ^  How  does  the  at- 
traction of  different  bodies  tend  to  affect  each  other's  motions  ?  What 
is  said  of  the  moon  1  When  does  the  sun  disturb  the  mutual  relations 


THE   PLANETS.  261 

the  earth  is,  is  more  attracted  than  the  earth  ;  and  at 
full  moon,  the  earth  being  nearer  the  sun  than  the  moon 
is,  is  more  attracted  than  the  moon.  Hence,  in  both 
cases,  the  sun  tends  to  separate  the  two  bodies.  At 
other  times,  as  when  the  rnoon  is  in  quadrature,  the 
influence  of  the  sun  tends  to  bring  the  bodies  nearer  to- 
gether. Sometimes  it  causes  the  moon  to  move  faster, 
and  sometimes  slower ;  so  that  owing  to  these  various 
causes,  the  moon's  motions  are  continually  disturbed, 
which  subjects  her  to  so  many  irregularities,  that  it 
has  required  vast  labor  and  research  to  ascertain  the 
exact  amount  of  each,  and  so  to  apply  it  as  to  assign 
the  precise  place  of  the  moon  in  the  heavens  at  any 
given  time. 

289.  Among  all  the  irregularities  to  which  the 
heavenly  bodies  are  subject,  there  is  not  one  which  the 
principle  of  universal  gravitation  does  not  account  for, 
and  even  render  necessary  ;  so  that  if  it  had  never 
been  actually  observed,  a  just  consideration  of  the  con- 
sequences of  the  operation  of  this  principle,  would 
authorize  us  to  say  that  it  must  take  place.  Indeed, 
many  of  the  known  irregularities  were  first  discovered 
by  the  aid  of  the  doctrine  of  gravitation,  and  afterward 
verified  by  actual  observation.  Such  a  tendency  of 
all  the  heavenly  bodies  to  disturb  each  other's  motions, 
might  seem  to  threaten  the  safety  of  the  whole  system, 
and  throw  the  whole  into  final  disorder  and  ruin  ;  but 
astronomers  have  shown,  by  the  aid  of  this  same  prin- 
ciple, that  all  possible  irregularities  which  can  occur 
among  the  planets,  have  a  narrow,  definite  limit — in- 
creasing first  on  one  side,  then  on  the  other,  and  thus 

of  the  moon  and  earth  1  When  does  the  sun  attract  the  moon  more 
than  the  earth7  When  the.  earth  more  than  the  moon  1  What  va- 
rious disturbances  does  it  produce  on  the  moon's  motions  1 

289.  Does  the  principle  of  universal  gravitation  account  for  the 
irregularities  of  the  celestial  motions  7  How  were  many  of  them 
first  discovered  1  Will  these  irregularities  produce  disorder  and 
ruin  1  What  has  been  shown  respecting  their  limit  1 


263  ASTRONOMY. 

vibrating  for  ever  about  a  mean  value,  which  secures 
the  stability  of  the  universe. 


CHAPTER    VII. 
COMETS. 

DESCRIPTION MAGNITUDE    AND     BRIGHTNESS PERIODS QUANTITY 

OF    MATTER MOTIONS PREDICTION     OF   THEIR    RETURNS DAN- 
GERS. 

290.  NOTHING  in  astronomy  is  more  truly  admirable, 
than  the  knowledge  which  astronomers  have  acquired 
of  the  motions  of  comets,  and   the  power  they   have 
gained  of  predicting  their  return.     Indeed,  everything 
belonging  to  this  class  of  bodies  is  so  wonderful,  as  to 
seem  rather  a  tale  of  romance  than  a  simple  recital  of 
facts. 

291.  A  comet,  when   perfectly  formed,  consists  of 
three  parts,  the  nucleus,   the  envelope,  and  the  tail. 
The  nucleus,  or  body  of  the  comet,  is  usually  distin- 
guished by  its  forming  a  bright  point  in  the  center  of 
the  head,  conveying  the  idea  of  a  solid,  or  at  least  of 
a  dense  portion  of  matter.     Though  it  is  usually  very 
small  when  compared  with  the  other  parts  of  the  comet, 
and  is  sometimes  wanting  altogether,  yet  it  occasion- 
ally is  large  enough  to  be  measured  by  the  aid  of  the 
telescope.     The  envelope  (sometimes  called  the  co-ma, 
from  a  Latin  word  signifying  hair,  in  allusion  to  its 
hairy  appearance,)  is  a  thick,  misty  covering,  that  sur- 
rounds the  head  of  the  comet.     Many  comets  have  nqr 
nucleus,  but  present  only  a  foggy  mass.     Indeed,  there 
is  a  regular  gradation  of  comets,  from  such  as  are  com- 
posed merely  of  a  gaseous  or  vapory  medium,  to  those 

290.  What  is  said  of  the  knowledge  astronomers  have  gained 
of  comets  1 

291.  Specify  the  several  parts  of  a  comet,  and  describe  each  part— 


COMETS.  263 

which  have  a  well-defined  nucleus.  In  some  instances, 
astronomers  have  detected,  with  their  telescopes,  small 
stars  through  the  densest  part  of  the  comet.  The  tail 
is  regarded  as  an  expansion  or  prolongation  of  the  en- 
velope, and  presenting,  as  it  sometimes  does,  a  train 
of  astonishing  length,  it  confers  on  this  class  of  bodies 
their  peculiar  celebrity.  These  several  parts  are  ex- 
hibited in  Fig.  121,  which  represents  the  appearance 

Fig.  121. 


of  the  celebrated  comet  of  1680,  and  which,  in  general 
size  and  shape,  is  not  unlike  that  of  1843.  The  latter, 
however,  was  not  so  broad  in  proportion  to  its  length, 
and  its  head  (including  the  nucleus  and  coma)  was  far 
less  conspicuous. 

292.  In  magnitude  and  brightness,  comets  exhibit 
great  diversity.  History  informs  us  of  several  comets 
so  bright  as  to  be  distinctly  visible  in  the  daytime, 
even  at  noon,  and  in  the  brightest  sunshine.  Such 
was  the  comet  seen  at  Rome  a  little  before  the  assas- 

the  nucleus— the  envelope — the  tail.  How  did  the  comet  of  1680 
compare  with  that  of  18431 

292.  What  is  said  of  the  magnitude  and  brightness  of  comets  7 
Of  the  comet  seen  at  Rome  1  Of  that  of  16SO 1  Of  1811 1  How 


264  ASTRONOMY. 

sination  of  Julius  Caesar ;  and,  in  a  superstitious  age, 
very  naturally  considered  as  the  precursor  of  that  event. 
The  comet  of  1680  covered  an  arc  of  the  heavens  of 
ninety-seven  degrees,  sufficient  to  reach  from  the  set- 
ting sun  to  the  zenith,  and  its  length  was  estimated  at 
123,000,000  miles.  The  comet  of  1811  had  a  nucleus 
only  428  miles  in  diameter,  but  a  tail  132,000,000 
miles  long ;  and  had  it  been  coiled  around  the  earth 
like  a  serpent,  it  would  have  reached  round  more  than 
5000  times.  Other  comets  are  exceedingly  small,  the 
nucleus  being  in  one  case  estimated  at  only  25  miles ; 
and  some  which  are  destitute  of  any  perceptible  nu- 
cleus, appear  to  the  largest  telescopes,  even  when  near- 
est to  us,  only  as  a  small  speck  of  fog.  The  majority 
of  comets  can  be  seen  only  by  the  aid  of  the  telescope. 
Indeed,  the  same  comet  has  different  appearances  at 
its  different  returns.  Halley's  comet,  in  1305,  was 
described  by  the  historians  of  that  age  as  the  comet  of 
"  horrific  magnitude ;"  yet,  in  1835,  when  it  reap- 
peared, the  greatest  length  of  its  tail  was  only  about 
twelve  degrees,  whereas  that  of  the  comet  of  1843  was 
about  forty  degrees. 

293.  The  periods  of  comets,  in  their  revolutions 
around  the  sun,  are  equally  various.  Encke's  comet, 
which  has  the  shortest  known  period,  completes  its 
revolution  in  3-J-  years  ;  while  that  of  1811  is  estimated 
to  have  a  period  of  3,383  years.  The  distances  to 
which  different  comets  recede  from  the  sun  are  equally 
various.  While  Encke's  comet  performs  its  entire 
revolution  within  the  orbit  of  Jupiter,  Halley's  comet 
recedes  from  the  sun  to  twice  the  distance  of  Uranus, 
or  3600,000,000  miles.  Some  comets,  indeed,  are 
thought  to  go  a  much  greater  distance  from  the  sun 

small  are  some  comets  1  How  does  the  same  comet  appear  at  its 
different  returns  1 

293.  What  is  said  of  the  periods  of  the  comets'?  Of  Encke's 
comet  1  Of  that  of  1811 1  What  of  the  distances  to  which  they 
recede  from  the  sun  1 


COMETS.  265 

than  this;  while  some  are  supposed  to  pass  into  curves, 
which  do  not,  like  the  ellipse,  return  into  themselves ; 
and,  in  this  case,  they  never  come  back  to  the  sun. 

294.  Comets  shine  by  reflecting  the  light  of  the  sun. 
In  one  or  two  cases,  they  have  been  thought  to  exhibit 
distinct  phases,  like  the  moon,  and  experiments  made 
on  the  light  itself,  indicate  that  it  is  reflected  and  not 
direct  light.     The  tails  of  comets  extend  in  a  direct 
line  from  the  sun,  following  the  body  as  it  approaches 
that  luminary,  and  preceding  the  body  as  it  recedes 
from  it. 

295.  The  quantity  of  matter  in  comets  is  exceedingly 
small.     The  tails  consist  of  matter  so  light,  that  the 
smallest  stars  are  visible  through  them.     They  can 
only  be  regarded  as  masses  of  thin  vapor,  susceptible 
of  being  penetrated  through  their  whole  substance  by 
the  sunbeams,  and  reflecting  them  alike   from  their 
interior  parts  and  from  their  surfaces.     "  The  highest 
clouds  that  float  in  our  atmosphere,'*'  (says  a  great 
astronomer,  Sir  John  Herschel,)  "  must  be  looked  upon 
as  dense  and  massive  bodies  compared  with  the  filmy 
and  all  but  spiritual  texture  of  a  comet."     The  small 
quantity  of  matter  in  comets  is  proved  by  the  fact, 
that  they  have  at  times  passed  very  near  to  some  of 
the  planets,  without  disturbing  their  motions  in  any 
appreciable  degree.    As  the  force  of  gravity  is  always 
proportioned  to  the  quantity  of  matter,  were  the  density 
of  these  bodies  at  all  comparable  to  their  size,  on 
coming   near   one  of  the    planets,  they  would    raise 
enormous  tides,   and   perhaps   even  draw  the   planet 
itself  out  of  its  orbit.     But  the  comet  of  1770,  in  its 
way  to  the  sun,  got  entangled  among  the  satellites  of 
Jupiter,  and  remained  near  them  four  months  ;  yet  it 


294.  By  what  light  do  comets  shine  1  Do  they  ever  exhibit  phases  1 
What  is  the  direction  of  their  tails  1 

295.  Quantity  of  matter  in  comets'?    Extreme  thinness1?    What 
proofe  are  stated  to  show  their  small  quantity  of  matter  1    What  is 

23 


266  ASTRONOMY. 

did  not  perceptibly  change  their  motions.  The  same 
comet  also  came  very  near  to  the  earth  ;  so  that,  had 
its  quantity  of  matter  been  equal  to  that  of  the  earth, 
it  would,  by  its  attraction,  have  caused  the  earth  to  have 
revolved  in  an  orbit  so  much  larger  than  at  present,  as 
to  have  increased  the  length  of  the  year  two  hours  and 
forty-seven  minutes.  Yet  it  produced  no  sensible  effect 
on  the  length  of  the  year.  It  may,  indeed,  be  asked, 
what  proof  we  have  that  comets  have  any  matter,  and 
are  not  mere  reflexions  of  light  ?  The  answer  is, 
that  although  they  are  not  able,  by  their  own  force  of 
attraction,  to  disturb  the  motions  of  the  planets,  yet 
they  are  themselves  exceedingly  disturbed  by  the  action 
of  the  planets,  and  in  exact  conformity  with  the  laws 
of  universal  gravitation.  A  delicate  compass  may  be 
greatly  agitated  by  the  vicinity  of  a  mass  of  iron,  while 
the  iron  is  not  sensibly  affected  by  the  attraction  of  the 
needle. 

296.  The  motions  of  comets  are  the  most  wonderful 
of  all  their  phenomena.  When  they  first  come  into 
view,  at  a  great  distance  from  the  sun,  as  is  sometimes 
the  case,  they  make  very  slow  approaches  from  day  to 
day,  and  even,  in  some  cases,  advance  but  little  from 
week  to  week.  When,  however,  they  come  near  to  the 
sun,  their  velocity  increases  with  prodigious  rapidity, 
sometimes  exceeding  a  million  of  miles  an  hour  ;  they 
wheel  around  the  sun  like  lightning  ;  and  recede  again 
with  a  velocity  which  diminishes  at  the  same  rate  as 
it  before  increased.  We  have  seen  that  the  planets 
move  in  orbits  which  are  nearly  circular,  and  that 
therefore  they  always  keep  at  nearly  the  same  distance 
from  the  sun.  Not  so  with  comets.  Their  perihelion 
distance  is  sometimes  so  small  that  they  almost  graze 


said  of  the  comet  of  1770 1    What  proof  have  we  that  they  contain 
any  matter  1 

296.  What  is  said  of  the  motions  of  comets  1    What  is  the  shape 
of  their  orbits  1    Of  their  distance  from  the  sun  at  the  perihelion 


COMETS.  267 

his  surface,  while  their  aphelion  lies  far  beyond 
the  utmost  bounds  of  the  planetary  system,  towards 
the  region  of  the  stars.  This  was  the  case  with  the 
comet  of  1680,  and  the  same  is  probably  true  of  the 
wonderful  comet  of  1843.  But  irregular  as  are  their 
motions,  they  are  all  performed  in  exact  obedience 
to  the  great  law  of  universal  gravitation.  The  radius 
vector  always  passes  over  equal  spaces  in  equal  times ; 
the  greater  length  of  the  triangular  space  described 
at  the  aphelion,  where  the  motion  is  so  slow,  being 
compensated  by  the  greater  breadth  of  the  triangular 
space  swept  over  at  the  perihelion,  where  the  motion  is 
so  swift. 

297.  The  appearances  of  the  same  comet  at  different 
periods  of  its  return  are  so  various,  that  we  can  never 
pronounce  a  given  comet  to  be  the  same  with  one  that 
has  appeared  before,  from  any  peculiarities  in  its  form, 
size,  or  color,  since  in  all  these  respects  it  is  very 
different  at  different  returns  ;  but  it  is  judged  to  be  the 
same  if  its  path  through  the  heavens,  as  traced  among 
the  stars,  is  the  same.  If,  on  comparing  two  comets 
that  have  appeared  at  different  times,  they  both  moved 
in  orbits  equally  inclined  to  the  ecliptic  ;  if  they  crossed 
the  ecliptic  in  the  same  place  among  the  stars ;  if  they 
came  nearest  the  sun,  or  passed  their  perihelion,  in  the 
same  part  of  the  heavens  ;  if  their  distance  from  the 
sun  at  that  time  was  the  same  ;  and,  finally,  if  they 
both  moved  in  the  same  direction  with  regard  to  the 
signs,  lhat  is,  both  east,  or  both  west ;  then  we  should 
pronounce  them  to  be  one  and  the  same  comet.  But 
if  they  disagreed  in  more  or  less  of  these  particulars, 
we  should  say  that  they  were  not  the  same  but  different 
bodies. 

and  at  their  aphelion  1  Are  the  motions  of  a  comet  subject  to  the 
laws  of  gravitation  1 

297.  How  do  we  determine  that  a  comet  is  the  same  with  one 
that  has  appeared  before  1  Enumerate  the  several  particulars  in 
which  the  two  must  agree  1 


268  ASTRONOMY. 

298.  Having  established  the  identity  of  a  comet  with 
one  that  appeared  at  some  previous  period,  the  interval 
between  the  two  periods  would  either  be  the  time  of  its 
revolution,  or  some   multiple  or  aliquot  part  of  that 
time.     Should  we,  for  example,  find  a  present  comet 
to  be  identical  with  one  that  appeared  150  years  ago,  its 
period  might  be  either  150  or  75  years,  since  possibly 
it  might  have  returned  to  the  sun  twice  in  150  years, 
although  its  intermediate  return,  at  the  end  of  75  years, 
was  either  not  observed  or  not  recorded.     Hence  the 
method  of  predicting  the  return  of  a  comet  which  has 
once  appeared  requires,  first,  that  we  ascertain  with 
all   possible  accuracy  the  particulars  enumerated   in 
article  297,  which  are  called  the  elements  of  the  comet, 
and  then  compare  these  elements  with  those  of  other 
comets  as  recorded  in  works  on  this  subject.      The 
elements  of  about  130  comets   have  been  found  and 
registered  in  astronomical  works,  to  serve  for  future 
comparison,  but  three  only  have  their  periodic  times 
certainly  determined.      These  are  Halley's,   Biela's, 
and  Encke's  comets  ;  the  first  of  which  has  a  period 
of  75  or  76  years  ;  the  second,  of  6|  years ;  the  third, 
of  31  years. 

299.  Halley's  comet  is  the  most  interesting  of  these, 
and    perhaps,   on  all    accounts,   the   most   interesting 
member  of  the  solar  system.     It  was  the  first  whose 
return  was  predicted  with  success.     Having  appeared 
in  1682,  Dr.  Halley,  a  great  English  astronomer,  then 
living,  ascertained  that  its  elements  were  the   same 
with  one  that  had  appeared  several  times   before,  at 
intervals  corresponding  to  about  seventy-six  years,  and 
hence  pronounced  this  to  be  its  period,  and  predicted 

298.  When  the  identity  with  a  previous  comet  is  established,  ho\v 
do  we  learn  the  time  of  us  revolution  1    What  is  the  method  of  pre- 
dicting their  return  1    Of  how  many  comets  have  the  elements  been 
determined  1    How  many  have  their  periods  certainly  ascertained  1 

299.  What  is  said  of  Halley's  comet  1    What  prevented  Halley'a 
fixing  the  exact  moment  of  its  return  1  What  is  said  about  weighing 


COMETS.  269 

that  in  about  seventy-six  years  more,  namely,  the  lat- 
ter part  of  1758  or  the  beginning  of  1759,  it  would 
return.  It  did  so,  and  came  to  its  perihelion  on  the 
13th  of  March,  1759.  What  prevented  his  fixing  the 
exact  moment,  was  the  uncertainty  which  then  existed 
with  respect  to  the  effects  of  the  planets  in  disturbing 
its  motions.  Since,  in  passing  down  to  the  sun,  it  would 
have  to  cross  the  orbits  of  all  the  planets,  and  would 
come  near  to  some  of  them,  it  was  liable  thus  to  be 
greatly  retarded  in  its  movements  by  the  powerful  at- 
traction of  these  great  bodies.  Before  the  exact  amount 
of  this  force  could  be  estimated,  the  precise  quantity 
of  matter  in  those  bodies  must  be  known  ;  that  is,  they 
must  be  weighed.  This  had  been,  at  that  time,  imper- 
fectly done.  It  has  since  been  done  with  the  greatest 
accuracy ;  such  large  bodies  as  Jupiter  and  Saturn 
have  been  weighed  as  truly  and  exactly  as  merchan- 
dise is  weighed  in  scales.  Hence,  on  the  late  return 
of  Halley's  comet,  in  1835,  the  precise  effect  of  all 
these  disturbing  forces  was  calculated,  and  the  time 
of  its  return  to  the  perihelion  assigned  to  the  very 
day. 

300.  The  success  of  astronomers  in  this  prediction 
was  truly  astonishing.  During  the  greatest  part  of  this 
long  period  of  seventy-six  years,  the  body  had  been 
wholly  out  of  sight,  beyond  the  planetary  system,  and 
beyond  the  reach  of  the  largest  telescopes.  It  must 
be  followed  through  all  this  journey  to  the  distance  of 
3600,000,000  of  miles  from  the  sun ;  and,  before  the 
precise  time  of  its  reappearance  could  be  predicted, 
the  amount  of  all  the  causes  that  could  disturb  its  mo- 
tions, arising  from  the  various  attractions  of  the  plan- 
ets, must  be  determined  and  applied.  Since,  moreover, 
these  forces  would  vary  with  every  variation  of  the 

the  planets'?  How  were  the  predictions  respecting  Halley's  comet 
fulfilled  in  18351 

300.  What  is  said  of  the  success  of  astronomers  in  this  prediction  1 
23* 


270  ASTRONOMY. 

distance,  the  calculation  was  to  be  made  for  every  de- 
gree of  the  orbit,  separately,  through  360  degrees,  for 
a  period  of  seventy-six  years.  Guided,  however,  by 
such  an  unerring  principle  as  universal  gravitation, 
astronomers  felt  no  doubt  that  the  comet  would  be  true 
to  its  appointed  time,  and  they  therefore  told  us,  months 
beforehand,  the  time  and  manner  of  its  first  approach, 
and  its  subsequent  progress.  They  told  us  that  early 
in  August,  1835,  the  comet  would  appear  to  the  tele- 
scope as  a  dim  speck  of  fog,  at  a  certain  hour  of  the 
night,  in  the  northeast,  not  far  from  the  seven  stars ; 
that  it  would  slowly  approach  us,  growing  brighter  and 
larger,  until,  in  about  a  month,  it  would  become  visible 
to  the  naked  eye ;  that,  on  the  night  of  the  7th  of  Oc- 
tober, it  would  approach  the  constellation  of  the  Great 
Bear,  and  move  along  the  northern  sky  through  the 
seven  bright  stars  of  that  constellation  called  the  Dip- 
per ;  that  it  would  pass  the  sun  about  the  middle  of 
November,  and  reappear  again  on  the  other  side  of 
the  sun  about  the  end  of  December.  All  these  pre- 
dictions were  verified,  with  a  degree  of  exactness  that 
constitutes  this  one  of  the  highest  achievements  of 
science. 

301.  Since  comets  which  approach  very  near  the 
sun,  like  the  comets  of  1680  and  1843,  cross  the  or- 
bits of  all  the  planets,  in  going  to  the  sun  and  return, 
ing,  the  possibility  that  one  of  them  may  strike  the  earth 
has  often  been  suggested,  and  at  times  created  great 
alarm.  It  may  quiet  our  apprehensions  on  this  subject 
to  reflect  on  the  vast  extent  of  the  planetary  spaces, 
in  which  these  bodies  are  not  crowded  together  as  we 
see  them  erroneously  represented  in  orreries  and  dia- 
grams, but  are  sparsely  scattered  at  immense  distances 
from  each  other,  resembling  insects  flying  in  the  open 

Describe  the  difficulties  attending  it.    What  did  astronomers  tell  us 
beforehand  1    How  were  these  predictions  fulfilled  1 
301.  What  is  said  of  the  danger  that  a  comet  will  strike  the  earth  1 


FIXED   STARS.  271 

heaven.  Such  a  meeting  with  the  earth  is  a  very  im- 
probable event ;  and  were  it  to  happen,  so  extremely 
light  is  the  matter  of  comets,  that  it  would  probably 
be  stopped  by  the  atmosphere ;  and  if  the  matter  is 
combustible,  as  we  have  some  reason  to  think,  it  would 
probably  be  consumed  without  reaching  the  earth. 
And,  finally,  notwithstanding  all  the  evils  of  which 
comets,  in  different  ages  of  the  world,  have  been  con- 
sidered as  the  harbingers,  we  have  no  reason  to  think 
that  they  ever  did  or  ever  will  do  the  least  injury  to 
mankind. 


CHAPTER   VIII. 
FIXED  STARS. 

NUMBER,    CLASSIFICATION,    AND    DISTANCE    OF   THE    STARS DIFFER- 
ENT   GROUPS    AND    VARIETIES NATURE    OF   THE    STARS,   AND    THg 

SYSTEM    OF   THE    WORLD. 

302.  VAST  as  are  the  dimensions  of  the  Solar  Sys- 
tem, to  which  our  attention  has  hitherto  been  confined, 
it  is  but  one  among  myriads  of  systems  that  compose 
the  Universe.  Every  star  is  a  world  like  this.  The 
fixed  stars  are  so  called,  because,  to  common  observa- 
tion, they  always  maintain  the  same  situations  with  re- 
spect to  each  other.  In  order  to  obtain  as  clear  and 
distinct  ideas  of  them  as  we  can,  we  will  consider,  un- 
der different  heads,  the  number,  classification,  and  dis- 
tances of  the  stars — their  various  orders — their  nature — 
and  their  arrangement  in  one  grand  system. 

SEC.  1.  Of  the  Number,  Classification,  and  Distances 
of  the  Stars. 

What  would  happen  if  it  should  1    Have  comets  ever  been  known 
to  do  any  injury  1 

302.  Why  are  the  fixed  stars  so  called  1  Under  what  different 
heads  ,are  the  fixed  stars  considered  1 


272  ASTRONOMY. 

303.  When  we  look  at  the  firmament  on  a  clear 
winter's  night,  the  number  of  stars  visible  even  to  the 
naked  eye,  seems  immense.     But  when  we  actually 
begin  to  count  them,  we  are  surprised  to  find  the  num- 
ber so  small.     In  some  parts  of  the  heavens,  half  a 
dozen  stars  will  occupy  a  large  tract  of  the  sky,  al- 
though in  other  parts  they  are  more  thickly  crowded 
together.     Hipparchus   of  Rhodes,  in  ancient   times, 
first  counted  the  stars,  and  stated  their  number  at  1022. 
If  we  stand  on  the  equator,  where  we  can  see  both  the 
northern  and  southern  hemispheres,  and  carefully  enu- 
merate the  stars  that  come  into  view  at  all  seasons  of 
the  year,  the  entire  number  will  amount  to  3000.     The 
telescope,  however,  brings  to  view  hosts  of  stars  in- 
visible to  the  naked  eye,  the  number  increasing  with 
every  increase  of  power  in  the  instrument ;  so  that  we 
may  pronounce  the  number  of  stars  that  are  actually 
distributed  through  the  fields  of  space,  to  be  literally 
endless.     Single  groups  of  half  a  dozen  stars,  as  seen 
by  the  naked  eye,  often  appear  to  a  powerful  telescope 
in  the  midst  of  hundreds  of  others   of  feebler  light. 
Astronomers  have  actually  registered  the  positions  of 
no  less  than  50,000 ;  and  the  whole  number  visible  in 
the  largest  telescopes  amounts  to  many  millions. 

304.  The  stars  are  classed  by  their  apparent  mag- 
nitudes.    The  whole  number  of  magnitudes  recorded 
is  sixteen,  of  which  the  first  six  only  are  visible  to  the 
naked  eye  ;  the  rest  are  telescopic  stars.     These  mag- 
nitudes are  not  determined  by  any  very  definite  scale, 
but  are  merely  ranked  according  to  their  relative  de- 
grees of  brightness,  and  this  is  left  in  a  great  measure 
to  the  judgment  of  the  eye  alone.     The  brightest  stars, 

303.  Apparent  number  of  the  stars  on  a  general  view.     Result 
when  we  count  them.  Who  first  made  a  catalogue  of  the  stars  1  How 
many  were  included  *      What  is  the  greatest  number  visible  to  the 
naked  eye  1    Numbers  visible  in  the  telescope  1    Whole  number  1 

304.  How  are  the  stars  classed  1    How  many  magnitudes  *?    How 
many  of  them  are  visible  to  the  naked  eye  1  What  are  the  rest  called  I 


FIXED  STARS.  273 

to  the  number  of  fifteen  or  twenty,  are  considered  as 
stars  of  the  first  magnitude  ;  the  fifty  or  sixty  next 
brightest,  of  the  second  magnitude ;  the  next  two 
hundred,  of  the  third  magnitude;  and  thus  the  number 
of  each  class  increases  rapidly,  as  we  descend  the 
scale,  so  that  no  less  than  fifteen  or  twenty  thousand 
are  included  within  the  first  seven  magnitudes. 

305.  The  stars  have  been  grouped  in  constellations 
from  the  most  remote  antiquity.  A  few,  as  Orion, 
Bootes,  and  Ursa  Major,  (the  Great  Bear,)  are  men- 
tioned in  the  most  ancient  writings,  under  the  same 
names  as  they  have  at  present.  The  names  of  the 
constellations  are  sometimes  founded  on  a  supposed 
resemblance  to  the  objects  to  which  those  names  be- 
long ;  as  the  Swan  and  the  Scorpion  were  evidently 
so  denominated  from  their  likeness  to  these  animals. 
But,  in  most  cases,  it  is  impossible  for  us  to  find  any 
reason  for  designating  a  constellation  by  the  figure  of 
the  animal  or  hero  which  is  employed  to  represent  it. 
These  representations  were  probably  once  connected 
with  the  fables  of  heathen  mythology.  The  same  fig- 
ures, absurd  as  they  appear,  are  still  retained  for  the 
convenience  of  reference ;  since  it  is  easy  to  find  any 
particular  star,  by  specifying  the  part  of  the  figure  to 
which  it  belongs ;  as  when  we  say  a  star  is  in  the 
neck  of  Taurus,  in  the  knee  of  Hercules,  or  in  the  tail 
of  the  Great  Bear.  This  method  furnishes  a  general 
clew  to  their  position  ;  but  the  stars  belonging  to  any 
individual  constellation,  are  distinguished  according 
to  their  apparent  magnitudes,  as  follows  :  First,  by  the 
Greek  letters,  Alpha,  Beta,  Gamma,  &c.  Thus,  Alpha, 
of  Orion,  denotes  the  largest  star  in  that  constellation ; 


How  many  stars  of  the  first  magnitude  1    How  many  of  the  second  1 
Of  the  third  1    How  many  within  the  first  seven  1 

305.  What  is  said  of  the  antiquity  of  the  constellations'?  Origin 
of  their  names  1  Why  are  the  ancient  figures  retained  1  How  are 
the  individual  stars  of  a  constellation  denoted  1 


274  ASTRONOMY. 

Beta,  of  Andromeda,  the  second  star  in  that;  and 
Gamma,  of  the  Lion,  the  third  brightest  star  in  the 
Lion.  When  the  number  of  the  Greek  letters  is  insuf- 
ficient, recourse  is  had  to  the  letters  of  the  Roman 
alphabet,  a,  b,  c,  &c.  ;  and  in  all  cases  where  these 
are  exhausted,  the  final  resort  is  to  numbers.  This 
will  evidently  at  length  become  necessary,  since  the 
largest  constellations  contain  many  hundreds  or  even 
thousands  of  stars. 

306.  When  we  look  at  the  firmament  on  a  clear 
Autumnal  or  Winter  evening,  it  appears  so  thickly  set 
with  stars,  that  one  would  perhaps  imagine,  that  the 
task  of  learning  even  the  brightest  of  them  would  be 
almost  hopeless.  So  far  is  this  from  the  truth,  that  it 
is  a  very  easy  task  to  become  acquainted  with  the 
names  and  positions  of  the  stars  of  the  first  magnitude, 
and  of  the  leading  constellations.  It  is  'but,  at  first, 
to  obtain  the  assistance  of  an  instructor,  or  some  friend 
who  is  familiar  with  the  stars,  just  to  point  out  a  few 
of  the  most  conspicuous  constellations.  A  few  of  the 
largest  stars  in  it  will  serve  to  distinguish  a  constella- 
tion, and  enable  us  to  recognise  it.  These  we  may 
learn  first,  and  afterward  fill  up  the  group  by  finding 
its  smaller  members.  Thus  we  may  at  first  content 
ourselves  with  learning  to  recognise  the  Great  Bear,  by 
the  seven  bright  stars  called  the  Dipper;  and  we  might 
afterward  return  to  this  constellation,  and  learn  to 
trace  out  the  head,  the  feet,  and  other  parts  of  the  ani- 
mal. Having  learned  to  recognise  the  most  noted  of 
the  constellations,  so  as  to  know  them  the  instant  we 
see  them  anywhere  in  the  sky,  we  may  then  learn 
the  names  and  positions  of  a  few  single  stars  of  special 
celebrity,  as  Sirius,  (the  Dog-Star,)  the  brightest  of  all 
the  fixed  stars,  situated  in  the  constellation  Canis  Ma- 


306.  Is  it  a  difficult  task  to  learn  the  constellations,  and  the  names 
of  the  largest  stars  1    What  directions  are  given  1 


FIXED  STARS.  275 

jor,  (the  Greater  Dog;)  Aldebaran,  in  Taurus ;  Arc- 
turus,  in  Bootes ;  Antares,  in  the  Scorpion ;  Capella,  in 
the  Wagoner. 

307.  It  is  a  pleasant  evening  recreation  for  a  small 
company  of  young  astronomers  to  go  out  together,  and 
learn  one  or  two  constellations  every  favorable  eve- 
ning, until  the  whole  are  mastered.     A  map  of  the 
stars,  placed  where  the  company  can  easily  resort  to 
it,  will,  by  a  little  practice,  enable  them  to  find  the 
relative  situations  of  the  stars,  with  as  much  ease  as 
they  find  those  of  places  on  the  map  of  any  country.     A 
celestial  globe,  when  it  can  be  procured,  is  better  still ; 
for  it  may  be  so  rectified  as  to  represent  the  exact 
appearance  of  the  heavens  on  any  particular  evening. 
It  will  be  advisable  to  learn  first  the  constellations  of  the 
zodiac,  which  have  the  same  names  as  the  signs  of 
the  zodiac  enumerated  in  Article  203,  (Aries,  Taurus, 
Gemini,  &c. ;)  although  any  order  may  be  pursued 
that  suits  the  season  of  the  year.     The  most  brilliant 
constellations  are  in  the  evening  sky  in  the  Winter.* 

308.  Great  difficulties  have  attended  the  attempt  to 
measure  the  distances  of  the  fixed  stars.     We  must 
here  call  to  mind  the  manner  in  which  the  distances 
of  nearer  bodies,  as  the  moon  and  the  sun,  are  ascer- 
tained, by  means  of  parallax.     The  moon,  for  exam- 
ple, is   at  the   same   moment   projected   on   different 
points  of  the  sky,  by  spectators  viewing  her  at  places 
on  the  earth  at  a  distance    from  each   other.     (See 
Art.  213.)     By  means  of  this  apparent  change  of  place 
in  the  moon,  when  viewed  from  different  places,  astron- 

*  For  more  particular  directions  for  studying  the  constellations,  inclu- 
ding a  description  of  the  most  important  of  them,  the  author  begs  leave  to 
refcr  to  his  larger  books,  as  the  "  School  Astronomy,"  and  "  Letters  on  As- 
tronomy." 

307.  What  is  proposed  as  an  evening's  recreation  1    What  use  is 
to  be  made  of  a  celestial  map  or  globe  1    With  what  constellations 
is  it  advisable  to  commence  1 

308.  What  is  said  of  the  attempt  to  measure  the  distances  of  the 


276  ASTRONOMY. 

omers,  as  already  explained,  derive  her  horizontal  par- 
allax,  and  from  that  her  distance  from  the  center  of 
the  earth.  The  stars,  however,  are  so  far  off,  that 
they  have  no  horizontal  parallax,  but  appear  always 
in  the  same  direction,  whether  viewed  from  one  part 
of  the  earth  or  another.  They  have  not,  indeed,  until 
very  recently,  appeared  to  have  any  annual  parallax  ; 
by  which  is  meant,  that  they  do  not  shift  their  places 
in  the  least  in  consequence  of  our  viewing  them  at 
different  extremities  of  the  earth's  orbit, — a  distance 
of  190,000,000  of  miles.  The  earth,  in  its  annual 
revolution  around  the  sun,  must  be  so  much  nearer  to 
certain  stars  that  lie  on  one  side  of  her  orbit,  than  she 
is  to  the  same  stars  when  on  the  opposite  side  of  her 
orbit ;  and  yet  even  this  immense  change  in  the  place- 
of  the  spectator,  makes  no  apparent  change  in  the- 
position  of  the  stars  of  the  first  magnitude  ;  which-,, 
from  their  being  so  conspicuous,  were  naturally  infer- 
red to  be  nearest  to  us.  Although  this  result  does  not 
tell  us  how  far  off  the  stars  actually  are,  yet  it  shows-- 
us  that  they  cannot  be  within  a  distance  of  twenty 
millions  of  millions  of  miles  ;  for  were  they  within 
that  distance,  the  nicest  observations  would  detect  in 
them  some  annual  parallax.  If  these  conclusions  are 
drawn  with  respect  to  the  largest  of  the  fixed  stars> 
which  we  suppose  to  be  vastly  nearer  to  us  than 
those  of  the  smallest  magnitude,  the  idea  of  distance 
swells  upon  us  when  we  attempt  to  estimate  the  re- 
moteness of  the  latter.  Of  some  stars  it  is  said,  that 
thousands  of  years  would  be  required  for  their  light  to 
travel  down  to  us. 

309.  By  some  recent  observations,  however,  it  is 
supposed  that  the  long  sought  for  parallax  among  the 
fixed  stars  has  been  discovered.  In  the  year  1838, 

fixed  stars'?  Have  the  stars  in  general  any  horizontal  parallax? 
What  is  meant  by  saying  that  the  stars  have  no  annual  parallax  1 
Beyond  what  distance  must  the  great  body  of  the  stars  be  1 


FIXED   STARS.  277 

Professor  Bessel,  of  Koningsberg,  (Prussia,)  announced 
the  discovery  of  a  parallax  in  one  of  the  stars  of  the 
constellation  Swan,  (61  Cygni,)  amounting  to  about 
one  third  of  a  second.  This  seems,  indeed,  so  small  an 
angle,  that  we  might  have  reason  to  suspect  the  reality 
of  the  deterrriination  ;  but  the  most  competent  judges, 
who  have  thoroughly  examined  the  process  by  which 
the  discovery  was  made,  give  their  assent  to  it.  What, 
then,  do  astronomers  understand  when  they  say,  that  a 
parallax  has  been  discovered  in  one  of  the  fixed  stars, 
amounting  to  one-third  of  a  second  ?  They  mean  that 
the  star  in  question  apparently  shifts  its  place  in  the 
heavens  to  that  amount,  when  viewed  at  opposite  ex- 
tremities of  the  earth's  orbit ;  namely,  at  points  in 
space  distant  from  each  other  190,000,000  of  miles. 
Let  us  reflect  how  small  an  arc  of  the  heavens  is  one- 
third  of  a  second  !  The  angular  breadth  of  the  sun  is 
but  small,  yet  this  is  toward  six  thousand  times  as 
great  as  the  discovered  parallax.  On  calculating  the 
distance  of  the  star  from  us,  by  this  means,  it  is  found 
to  be  six  hundred  and  fifty-seven  thousand  seven 
hundred  times  ninety-five  millions  of  miles, — a  dis- 
tance which  it  would  take  Jight  more  than  ten  years  to 
traverse. 

SEC.  2.  Of  Groups  and  Varieties  of  Stars. 

310.  Under  this  head,  we  may  consider  Double, 
Temporary,  and  Variable  Stars  ;  Clusters  and  Nebu- 
lae. Double  Stars  are  those  which  appear  single  to  the 
naked  eye,  but  are  resolved  into  two  by  the  telescope  ; 
or,  if  not  visible  to  the  naked  eye,  they  are  such  as, 


309.  Give  an  account  of  the  discovery  of  the  parallax  of  61  Cygni. 
How  much  is  it  1   What  dp  astronomers  understand  by  this  1    How 
much  less  angular  breadth  is  one-third  of  a  second  than  the  breadth 
of  the  sun  1    What  distance  does  this  imply  1 

310.  Enumerate  the  different  groups  and  varieties  of  the  stars. 

24 


278 


ASTRONOMY. 


when  seen  in  the  telescope,  are  so  close  together  as  to 
be  regarded  as  objects  of  this  class.  Sometimes,  three 
or  more  stars  are  found  in  this  near  connection,  consti- 
tuting triple  or  multiple  stars.  Castor,  for  example, 
(one  of  the  two  bright  stars  in  the  constellation  Gemi- 
ni,) when  seen  by  the  naked  eye,  appears  as  a  single 
star  ;  but  in  a  telescope,  even  of  moderate  power,  it  is 
resolved  into  two.  These  are  nearly  of  equal  size  ; 
but,  more  commonly,  one  is  exceedingly  small  in  com- 
parison with  the  other,  resembling  a  satellite  near  its 
primary,  although  in  distance,  in  light,  and  in  other 
characteristics,  each  has  all  the  attributes  of  a  star, 
and  the  combination,  therefore,  cannot  be  that  of  a  star 

Fig.  122. 


with  a  planetary  satellite.     The  diagram  shows  four 
double  stars,  as  they  appear  in  large  telescopes. 

311.  A  circumstance  which  has  given  great  interest 
to  the  double  stars  is,  the  recent  discovery  that  some 
of  them  revolve  around  each  other.  Their  times  of 
revolution  are  very  different,  varying  in  the  case  of 
those  already  ascertained,  from  43  to  1000  years,  or 
more.  The  revolutions  of  these  stars  have  revealed  to 
us  this  most  interesting  fact,  that  the  law  of  gravitation 


What  are  double  stars'?    Give  an  example  in  Castor.    Why  may 
not  the  smaller  star  be  a  planetary  satellite  1 


FIXED   STARS.  279 

extends  to  the  fixed  stars.  Before  these  discoveries,  we 
could  not  decide,  except  by  a  feeble  analogy,  that  this 
law  extended  beyond  the  bounds  of  the  solar  system. 
Indeed,  our  belief  rested  more  upon  our  idea  of  unity 
of  design  in  the  works  of  the  Creator,  than  upon  any 
certain  proof;  but  the  revolution  of  one  star  around 
another,  in  obedience  to  forces  which  are  proved  to  be 
similar  to  those  which  govern  the  solar  system,  estab- 
lishes the  grand  conclusion,  that  the  law  of  gravitation 
is  truly  the  law  of  the  material  universe. 

312.  Temporary  Stars  are  new  stars,  which  have 
appeared  suddenly  in  the  firmament,  and  after  a  certain 
interval,   as  suddenly  disappeared,   and   returned    no 
more.     It  was  the  appearance  of  a  new  star  of  this 
kind,  one  hundred  and  twenty-five  years  before  the 
Christian  era,  that  prompted  Hipparchus  to  draw  up  a 
catalogue  of  the    stars,   so  that    future    astronomers 
might  be  able  to  decide  the  question,  whether  the  starry 
heavens  are  unchangeable  or  not.     Such,  also,  was 
the  star  which  suddenly  shone  out  in  the  year  389,  in 
the  constellation  Eagle,  as  bright  as  Venus,  and  after 
remaining  three  weeks,  disappeared  entirely.     In  1572, 
a  new  star  suddenly  appeared,  as  bright  as  Sirius,  and 
continued  to  increase  until  it  surpassed  Jupiter  when 
brightest,  and  was  visible  at  mid-day.     In  a  month,  it 
began  to  diminish  ;  and,  in  three  weeks  afterward,  it 
entirely  disappeared.     It  is  also  found  that  stars  are 
now  missing,  which  were  inserted  in  ancient  catalogues, 
as  then  existing  in  the  heavens. 

313.  Variable  Stars  are  those  which  undergo  a  pe- 
riodical change  of  brightness.     One  of  these  is  the 
star  Mira,  in  the  whale.     It  appears  once  in  eleven 


311.  What  has  recently  given  great  interest  to  the  double  stars'? 
What  inference  is  made  respecting  the  law  of  gravitation  1 

312.  What  are  temporary  stars  1    What  led  Hipparchus  to  num- 
ber the  stars  1    What  is  said  of  the  star  of  389 '<    Of  1572 1    What 
etars  are  now  missing  1 


280  ASTRONOMY. 

months,  remains  at  its  greatest  brightness  about  a  fort- 
night, being  then  equal  to  a  star  of  the  second  magni- 
tude. It  then  decreases  about  three  months,  until  it 
becomes  completely  invisible,  and  remains  so  about 
five  months,  when  it  again  becomes  visible,  and  con- 
tinues increasing  during  the  remaining  three  months 
of  its  period.  Another  variable  star  in  Perseus,  goes 
through  a  great  variety  of  changes  in  the  course  of 
three  days.  Others  require  many  years  to  accomplish 
the  period  of  their  changes. 

314.  Clusters  of  stars  will  next  claim  our  attention. 
In  various  parts  of  the  sky,  in  a  clear  night,  are  seen 
large  groups  which,  either  by  the  naked  eye,  or  by 
the  aid  of  the  smallest  telescope,  are  perceived  to  con- 
sist of  a  great  number  of  small  stars.  Such  are  the 
Pleiades,  Coma  Berenices,  (Berenice's  Hair,)  and 
Prsesepe,  or  the  Beehive,  in  Cancer.  The  Pleiades, 
or  Seven  Stars,  as  they  are  called,  in  the  neck  of  Tau- 
rus, is  the  most  conspicuous  cluster.  With  the  naked 
eye,  we  do  not  distinguish  more  than  six  stars  in  this 
group  ;  but  the  telescope  exhibits  fifty  or  sixty  stars, 
crowded  together,  and  apparently  separated  from  the 
other  parts  of  the  starry  heavens.  Berenice's  Hair, 
which  may  be  seen  in  the  summer  sky  in  the  west,  a 
little  westward  of  Arcturus,  has  fewer  stars,  but  they 
are  of  a  larger  class  than  those  which  compose  the 
Pleiades.  The  Beehive,  or  Nebula  of  Cancer,  as  it  is 
called,  is  one  of  the  finest  objects  of  this  kind  for  a 
small  telescope.  A  common  spy-glass,  indeed,  is  suf- 
ficient to  resolve  it  into  separate  stars.  It  is  easily 
found,  appearing  to  the  naked  eye  somewhat  hazy, 
like  a  comet,  the  stars  being  so  near  together  that  their 
light  becomes  blended.  A  reference  to  a  celestial 
map  or  globe  will  show  its  exact  position  in  the  con- 

313.  What  are  variable  stars  !    Give  an  example  in  Mira,  and  in 
Perseus. 

314.  What  is  said  of  clusters  of  stars  1    Give  examples.    What  is 


FIXED   STARS.  281 

Stellation  Cancer,  and  it  will  well  repay  those  who  can 
command  a  telescope  of  any  size,  for  the  trouble  of 
looking  it  up.  A  similar  cluster  in  the  sword  handle 
of  Perseus,  near  the  well-known  object,  Cassiopea's 
Chair,  in  the  northern  sky,  also  presents  a  very  beau- 
tiful appearance  to  the  telescope. 

315.  Nebula  are  faint,  misty  appearances,  which 
are  dimly  seen  among  the  stars,  resembling  comets,  or 
a  speck  of  fog.  A  few  are  visible  to  the  naked  eye  ; 
one,  especially,  in  the  girdle  of  the  constellation 
Andromeda,  which  has  often  been  reported  as  a  newly 
discovered  comet.  The  greater  part,  however,  are 
visible  only  to  telescopes  of  greater  or  less  power. 
They  are  usually  resolved  by  the  telescope  into 
myriads  of  small  stars  ;  though,  in  some  instances,  no 
powers  of  the  telescope  have  been  found  sufficient  to 
resolve  them.  The  Galaxy,  or  Milky  Way,  presents 
a  continual  succession  of  large  nebulae.  The  great 
English  astronomer,  Sir  William  Herschel,  has  given 
catalogues  of  2,000  nebulae,  and  has  shown  that 
nebulous  matter  is  distributed  through  the  immensity 
of  space  in  quantities  inconceivably  great,  and  in 
separate  parcels  of  all  shapes  and  sizes,  and  of  all 
-degrees  of  brightness,  between  a  mere  milky  veil  and 
the  condensed  light  of  a  fixed  star.  In  fact,  more 
distinct  nebulae  have  been  hunted  out  by  the  aid  of 
telescopes,  than  the  whole  number  of  stars  visible  to  the 
naked  eye  in  a  clear  winter's  night.  Their  appearances 
are  extremely  diversified.  In  many  of  them  we  can 
easily  distinguish  the  individual  stars  ;  in  those 
apparently  more  remote,  the  interval  between  the  stars 
diminishes,  until  it  becomes  quite  imperceptible  ;  and 


said  of  the  Pleiades'?  What  of  Berenice's  Hair'?  What  of  the  Bee- 
hive 1    Of  the  cluster  in  Perseus'? 

315.  What  are  Nebula?  1  Are  any  visible  to  the  naked  eye  1  How 
do  they  appear  by  the  telescope  1    What  is  said  of  the  Galaxy  or 
Milky  Way  1    How  many  nebulas  did  Herschel  discover  1    Can  we 
24* 


282  ASTRONOMY. 

in  their  faintest  aspect  they  dwindle  to  points  so  minute, 
as  to  be  appropriately  called  star  dust.  Beyond  this,  no 
stars  are  distinctly  visible,  but  only  streaks  or  patches 
of  milky  light.  In  objects  so  distant  as  these  assem- 
blages of  stars,  any  apparent  interval  between  them 
must  imply  an  immense  distance  ;  and  were  we  to  take 
our  station  in  the  midst  of  them,  a  firmament  would 
expand  itself  over  our  heads  like  that  of  our  evening 
sky,  only  a  thousand  times  more  rich  and  splendid  ; 
and  were  we  to  take  our  view  from  such  a  distant  part 
of  the  universe,  it  is  thought  by  astronomers  that  our 
own  starry  heavens  would  all  melt  together  into  the 
same  soft  and  mysterious  light,  and  be  seen  as  a  faint 
nebula  on  the  utmost  verge  of  creation. 

316.  Many  of  the  nebulae  exhibit  a  tendency  toward 
a  globular  form,  and  indicate  a  rapid  condensation 
toward  the  center.  These  wonderful  objects,  however, 
are  not  confined  to  any  particular  form,  but  exhibit 
great  varieties  of  figure.  Sometimes  they  appear  of 
an  oval  form  ;  sometimes  they  are  shaped  like  a  fan  ; 
and  the  unresolvable  kind  often  assume  the  most 
fantastic  forms.  But,  since  objects  of  this  kind  must 
be  seen  before  they  can  be  fully  understood,  it  is  hoped 
the  learner  will  avail  himself  of  any  opportunity  he  may 
have  to  contemplate  them  through  the  telescope.  Some 
of  them  are  of  astonishing  dimensions.  It  is  but  little 
to  say  of  many  a  nebula,  that  it  would  more  than  cover 
the  whole  solar  system,  embracing  within  it  the  immense 
orbit  of  Uranus. 

SEC.  3.  Of  the  Nature  of  the  Stars,  and  the  System 
of  the  World. 


resolve  them  all  into  stars  "?  If  we  were  to  take  our  position  in  the 
midst  of  a  great  nebula,  what  should  we  see  over  our  heads  1  How 
would  our  firmament  appear  1 

316.  What  is  said  of  the  different  forms  of  nebulae  1     What  of 
their  dimensions  1 


FIXED   STARS.  283 

317.  We  have  seen  that  the  stars  are  so  distant,  that 
not  only  would  the  earth  dwindle  to  a  point,  and  entirely 
vanish  as  seen  from  the  nearest  of  them,  but  that  the  sun 
itself  would  appear  only  as  a  distant  star,  less  brilliant 
than  many  of  the  stars  appear  to  us.    The  diameter  of 
the  orbit  of  Uranus,  which  is  about  3600,000,000  of 
miles,  would,  as  seen  from  the  nearest  star,  appear  so 
small   that   the   finest   hair  would   more   than   cover 
it.     The  telescope  itself,  seems  to  lose  all  power  when 
applied  to  measure  the  magnitudes  of  the  stars  ;  for 
although  it  may  greatly  increase  their  light,  so  as  to 
make  them  dazzle  the  eye  like  the  sun,  yet  it  makes 
them  no  larger.     They  are  still  shining  points.     We 
may  bring  them,  in  effect,  6000  times  nearer,  and  yet 
they  are  still  too  distant  to  appear  otherwise  than  points. 
It  would,  therefore,  seem  fruitless  to  inquire  into  the 
nature  of  bodies  so  far  from  us,   and  which  reveal 
themselves  to  us  only  as  shining  points  in  space.     Still 
there  are  a  few  very  satisfactory  inferences  that  can  be 
made  out  respecting  them. 

318.  First,  the  fixed  stars  are  bodies  greater  than  our 
earth.      Were  the  stars  no  larger  than  the  earth,  it 
would  follow,  on  optical  principles,  that  they  could  not 
be  seen  at  such  a  distance  as  they  are.    Attempts  have 
been  made  to  estimate  the  comparative  magnitudes  of 
the  brightest  of  the  fixed  stars,  from  the  light  which 
they  afford.     Knowing  the  rate  at  which  the  intensity 
of  light  decreases  as  the  distance  increases,  we  can 
find  how  far  the  sun  must  be  removed  from  us,  in  order 
to  appear  no  brighter  than  Sirius.     The  distance  is 
found  to  be  140,000  times  its  present  distance.     But 
Sirius  is  more  than  200,000  times  as  far  off  as  the 
sun  ;  hence  it  is  inferred,  that  it  must,  upon  the  lowest 
estimate,  give  out  twice  as  much  light  as  the  sun  ;  or 

317.  How  would  our  sun  appear  from  the  nearest  fixed  star  1 
How  broad  would  the  orbit  of  Uranus  appear  *? 

318.  What  is  said  of  the  size  of  the  stars  1  Are  the  stars  of  various 


284  ASTRONOMY. 

that,  in  point  of  splendor,  Sirius  must  be  at  least  equal 
to  two  suns.  Indeed,  it  is  thought  that  its  light  equals 
that  of  fourteen  suns.  There  is  reason,  however,  to 
believe,  that  the  stars  are  actually  of  various  magni- 
tudes, and  that  their  apparent  difference  is  not  owing,  as 
some  have  supposed,  merely  to  their  different  distances. 
The  two  members  of  the  double  star  in  the  Swan,  (61 
Cygni,)  the  motion  of  one  of  which  has  led  to  the 
discovery  of  a  parallax,  (see  Art.  309,)  are  severally 
thought  to  have  less  than  half  the  quantity  of  matter  in 
the  sun,  which  accounts  for  their  appearing  so  diminutive 
in  size,  while  they  are  apparently  so  much  nearer  to  us 
than  the  great  body  of  the  stars. 

319.  Secondly,  the  fixed  stars  are  Suns.  It  is  inferred 
that  they  shine  by  their  own  light,  and  not  like  the 
planets,  by  reflected  light,  since  reflected  light  would 
be  too  feeble  to  render  them  visible  at  such  a  distance. 
Moreover,  it  can  be  ascertained  by  applying  certain 
tests  to  light  itself,  whether  it  is  direct  or  reflected 
light ;  and  the  light  of  the  stars,  when  thus  examined, 
proves  to  be  direct.  Since,  then,  the  stars  are  large 
bodies  like  the  sun  ;  since  they  are  immensely  farther 
off  than  the  farthest  planet ;  since  they  shine  by  their 
own  light ;  and,  in  short,  since  their  appearance  is, 
in  all  respects,  the  same  as  the  sun  would  exhibit  if 
removed  to  the  region  of  the  stars,  the  conclusion  is 
unavoidable  that  the  stars  are  suns.  We  are  justified, 
therefore,  by  sound  analogy,  in  concluding  that  the 
stars  were  made  for  the  same  end  as  the  sun ;  namely, 
as  the  centers  of  attraction  to  other  planetary  worlds, 
to  which  they  severally  afford  light  and  heat.  The 
chief  purpose  of  the  stars  could  not  have  been 
to  adorn  the  firmament,  or  to  give  light  by  night, 
since  by  far  the  greater  part  of  them  are  invisible  to 

magnitudes  1    How  large  are  the  two  members  of  the  double  star 
61  Cygni  1 
819.  How  is  it  shown  that  the  stars  are  suns  1  For  what  were  they 


FIXED   STARS.  285 

the  naked  eye  ;  nor  as  landmarks  to  the  navigator,  for 
only  a  small  portion  of  them  are  adapted  to  this  pur- 
pose  :  nor,  finally,  to  influence  the  earth  by  their  at- 
tractions, since  their  distance  renders  such  an  effect 
entirely  insensible.  If  they  are  suns,  and  if  they  ex- 
ert no  important  agencies  upon  our  world,  but  are 
bodies  evidently  adapted  to  the  same  purpose  as  our 
sun,  then  it  is  as  rational  to  suppose  that  they  were 
made  to  give  light  and  heat,  as  that  the  eye  was  made 
for  seeing  and  the  ear  for  hearing. 

320.  We  are  thus  irresistibly  led  to  the  conclusion, 
that  each  star  is  a  world  within  itself, — a  sun,  attend- 
ed, like  our  sun,  by  planets  to  which  it  dispenses  light 
and  heat,  and  whose  motions  it  controls  by  its  attrac- 
tion. Moreover,  since  we  see  all  things  on  earth  con- 
trived in  reference  to  the  sustenance,  safety,  and  hap- 
piness of  man, — the  light  for  his  eyes,  the  air  for  his 
lungs,  the  heat  to  warm  him,  and  to  perform  his  labors 
by  its  mechanical  and  chemical  agencies ;  since  we  see 
the  earth  yielding  her  flowers  and  fruits  for  his  sup- 
port, and  the  waters  flowing  to  quench  his  thirst,  or  to 
bear  his  ships,  and  all  the  animal  tribes  subjected  to 
his  dominion  ;  and,  finally,  since  we  see  the  sun  him- 
self endued  with  such  powers,  and  placed  at  just  such 
a  distance  from  him,  as  to  secure  his  safety  and  min- 
ister in  the  highest  possible  degree  to  his  happiness ; 
we  are  left  in  no  doubt  that  this  world  was  made  for 
the  dwelling  place  of  man.  But,  on  looking  upward 
at  the  other  planets,  when  we  see  other  worlds  resem- 
bling this  in  many  respects,  enlightened  and  regulated 
by  the  same  sun,  several  of  them  much  larger  than  the 
earth,  furnishing  a  more  ample  space  for  intelligent 
beings,  and  fitted  up  with  a  greater  number  of  moons 


made  1  Might  it  not  have  been  to  give  light  by  night — to  afibrd  land- 
marks to  the  navigator — or  to  exert  a  power  of  attraction  on  the  earth  1 
320.  To  what  conclusion  are  we  thus  led  1    For  what  end  were 
the  stars  made  1 


286  ASTRONOMY. 

to  give  them  light  by  night,  we  can  hardly  resist  the 
conclusion  that  they,  too,  are  intended  as  the  abodes  of 
intelligent,  conscious  beings,  and  are  not  mere  solitary 
wastes.  Finally,  the  same  train  of  reasoning  conducts 
us  to  the  conclusion,  that  each  star  is  a  solar  system, 
and  that  the  universe  is  composed  of  worlds  inhabited 
by  different  orders  of  intelligent  beings. 

321.  It  only  remains  to  inquire  respecting  the  Sys- 
tem of  the  World,  or  to  see  in  what  order  the  various 
bodies  that  compose  the  universe  are  arranged.  One 
thing  is  apparent  to  all  who  have  studied  the  laws  of 
nature, — that  great  uniformity  of  plan  attends  every 
department  of  the  works  of  creation.  A  drop  of  water 
has  the  same  constitution  as  the  ocean  ;  a  nut-shell  of 
air,  the  same  as  the  whole  atmosphere.  The  nests 
and  the  eggs  of  a  particular  species  of  birds  are  the 
same  in  all  ages  ;  the  anatomy  of  man  is  so  uniform, 
that  the  mechanism  of  one  body  is  that  of  the  race. 
A  similar  uniformity  pervades  the  mechanism  of  the 
heavens.  To  begin  with  the  bodies  nearest  to  us,  we 
see  the  earth  attended  by  a  satellite,  the  moon,  that 
revolves  about  her  in  exact  obedience  to  the  law  of 
universal  gravitation.  Since  the  discovery  of  the  tel- 
escope has  enabled  us  to  see  into  the  mechanism  of 
the  other  planets,  we  see  that  Jupiter,  Saturn,  and 
Uranus,  have  each  a  more  numerous  retinue,  but  all 
still  fashioned  according  to  the  same  model,  and  obe- 
dient to  the  same  law.  The  recent  discovery  of  the 
revolution  of  one  member  of  a  double  star  around  the 
other,  shows  that  the  same  organization  extends  to  the 
stars ;  and  certain  motions  of  our  own  sun  and  his 
attendant  worlds,  indicate  that  our  system  is  likewise 
slowly  revolving  around  some  other  system.  In  each 
of  the  clusters  of  stars  and  nebulae,  we  also  see  a  mul- 

321.  What  is  said  of  the  uniformity  of  plan  visible  in  the  works  ot 
nature  1  Show  that  a  similar  uniformity  prevails  in  the  general  plan 
of  the  celestial  bodies.  How  is  this  exemplified  in  the  systemsof  Ju- 
piter, Saturn,  and  Uranus  1  In  the  revolutions  of  double  stars  1  What 


FIXED  STARS.  287 

titude  of  stars  assembled  together  into  one  group ;  and, 
although  we  have  not  yet  been  able  to  detect  a  common 
system  of  motions  of  revolution  among  them,  and  on 
account  of  their  immense  distance,  particularly  of  the 
nebulae,  perhaps  we  never  shall  be  able,  yet  this  very 
grouping  indicates  a  mutual  relation,  and  the  symmet- 
rical forms  which  many  of  them  exhibit,  prove  an  or- 
ganization for  some  common  end.  Now  such  is  the 
uniformity  of  the  plan  of  creation,  that  where  we  have 
discovered  what  the  plan  is  in  the  objects  nearest  to  us, 
we  may  justly  infer  that  it  is  the  same  in  similar  ob- 
jects, however  remote.  Upon  the  strength  of  a  sound 
analogy,  therefore,  we  infer  revolutions  of  the  bodies 
composing  the  most  distant  nebulae,  similar  to  those 
which  we  see  prevail  among  all  nearer  worlds. 

322.  This  argument  is  strengthened  and  its  truth 
rendered  almost  necessary,  by  the  fact  that  without 
such  motions  of  revolution,  the  various  bodies  of  the 
universe  would  have  a  tendency  to  fall  into  disorder 
and  ruin.  By  their  mutual  attractions,  they  would  all 
tend  directly  toward  each  other,  moving  at  first,  in- 
deed, with  extreme  slowness,  but  in  the  lapse  of  ages, 
with  accelerated  velocity,  until  they  finally  rushed  to- 
gether in  the  common  center  of  gravity.  We  can  con- 
ceive of  no  way  in  which  such  a  consequence  could 
be  avoided,  except  that  by  which  it  is  obviated  in  the 
systems  which  are  subject  to  our  observation,  namely,  by 
a  projectile  force  impressed,  upon  each  body,  which 
makes  it  constantly  tend  to  move  directly  forward  in  a 
straight  line,  but  which,  when  combined  with  the  force 
of  gravity  existing  mutually  in  all  the  bodies  of  the 
system,  gives  them  harmonious  revolutions  around 
each  other. 


indications  of  systematic  arrangement  do  we  see  in  the  clusters  and 
nebulae  1 

322.  What  would  happen  to  the  various  bodies  in  the  universe  with- 
out such  revolutions!    How  could  such  a  consequence  be  avoided  1 


288  ASTRONOMY. 

323.  We  see,  then,  in  the  subordinate  members  of 
the  solar  system,  in  the  earth  and  its  moon,  in  Jupiter, 
Saturn,  and  Uranus,  with  their  moons,  a  type  of  the 
mechanism  of  the  world,  and  we  conclude   that   the 
material  universe  is  one  great  system ;  that  the  combi- 
nation of  planets  with  their  satellites,  constitutes  the 
first  or  lowest  order  of  worlds ;  that,  next  to  these, 
planets  are  linked  to  suns  ;  that  these  are  bound  to  other 
suns,  composing  a  still  higher  order  in  the  scale  of 
being;  and,  finally,  that  all  the  different  systems  of 
worlds  move  around  their  common  center  of  gravity. 

324.  The  view  which  the   foregoing  considerations 
present  to  us  of  the  grandeur  of  the  material  universe, 
is  almost  overwhelming;    and  we  can  hardly  avoid 
joining  in  the  exclamations  that  have  been   uttered, 
after  the   same  survey,  upon  the  insignificant  place 
which  we  occupy  in  the  scale  of  being,  nor  cease  to 
wonder,  with  Addison,  that  we  are  not  lost  among  the 
infinitude  of  the  works  of  God.     It  is  cause  of  devout 
thankfulness,  however,  that  omniscience  and  benevo- 
lence are  at  the  helm  of  the  universe ;  that  the  same 
hand  which  fashioned  these  innumerable  worlds,  and 
put  them  in  motion,  still  directs  them  in  their  least  as 
well  as  in  their  greatest  phenomena  ;  and  that,  if  such 
a  view  as  we  have  taken  of  the  power  of  the  Creator, 
fills  us  with  awe  and  fear,  the  displays  of  car6  mani- 
fested in  all  his  works  for  each  of  the  lowest  of  his 
creatures,  no  less  than  for  worlds  and  systems  of  worlds, 
should  conspire  with  what,  we  know  of  his  works  of 
Providence  and  Grace^  to  fill  us  with  love  and  adora- 
tion. 

323.  Describe  J;he  system  of  the  world. 

324.  What  is  said  of  the  grandeur  of  these  views  7   What  is  spe- 
cial cause  of  thankfulness  1    How  should  the  contemplation  of  the 
subject  affect  us  1 , 


YA  04362 


THE  UNIVERSITY  OF  CALIFORNIA  LIBRARY 


